ARRANGEMENT FOR EVAPORATING LIQUIDS

The present invention relates to an arrangement for evaporating liquids, which is carried out utilizing vapor compression evaporation. The arrangement can especially be applied in a black liquor evaporation plant of a chemical pulp mill, where pre-evaporation of black liquor is effected in a novel way using vapor compression evaporation.

For combustion in a recovery boiler, black liquor is concentrated most usually in a multi- effect evaporation plant. The numbering of the effects is based in the order defined by their vapor flow. Most usually an evaporation plant operates so that an evaporator under the highest pressure receives as a source of heat e.g. fresh steam, which boils liquid on the liquid-side of the evaporator, whereby vapor is generated having a lower pressure than the pressure of steam introduced to the steam-side of the evaporator. The generated vapor is used for heating an evaporator that is under the next lower pressure, as the liquor flows through the evaporation plant countercurrently in relation to the heating steam or vapor. Correspondingly, in a third effect, a fourth effect etc., the same process takes place, until the pressure and temperature of the vapor from the last effect in relation to the vapor flow are so low that it is not worth continuing both in technical sense and in view of the total economy. Vapor at the lowest pressure is usually condensed in a condenser with water or air. Typically an evaporation plant comprises 5-7 effects. One conventional evaporation unit type is a so-called falling film evaporator, where liquor is introduced to flow downwards on the surface of a heating element in form of a thin film in order to generate a high heating efficiency.

In black liquor concentration, for combustion, also vapor-compression evaporation has been used, which is also referred to as mechanical vapor recompression (MVR), where the pressure, and thus also the saturation temperature of the vapor evaporated from a solution is raised with steam by means of an ejector, or mechanically by means of a compressor or a fan so high that this vapor can be used as heating medium of the same effect. The compression is required in order to raise the saturation temperature of the vapor for generating an adequate temperature difference over the evaporation effect. The increase in the saturation temperature is to be greater than the boiling point rise. As the power demand of the compressor or fan is relative to the pressure increase, vapor compression evaporation has been used mostly when the boiling point rise is low. A re- stricting factor for mechanical vapor compression in black liquor evaporation is the black

liquor boiling point rise. As the dry solids content of the black liquor increases, the boiling point rise increases fast and reduces the available temperature difference. In practice, the use of fan evaporators in sulfate pulp industry is limited to pre-evaporators, where liquor is evaporated in approximately 20-25 % dry solids content. Pre-evaporation is typically followed by the above described multi-effect evaporation for increasing the dry solids content of the black liquor to a preferred dry solids content, typically 75-85 %, even 90 %, for combustion in the recovery boiler. Typically, falling film evaporators are suitable for use as compression evaporators, because they operate effectively also with small temperature differences.

The economical applicability of vapor compression evaporation is dependent on the fact that the price of electrical energy is low enough compared to the price of available heat energy. An advantage of this evaporation method is that no external steam source is required.

Finnish patent publication 51835 (US 3475281) describes an evaporation plant, where e.g. black liquor is first evaporated in a vapor compression evaporation plant, where vapors being discharged from the evaporator are compressed with a compressor and returned to the same effect as heat source. In a second evaporation effect the liquor is fur- ther concentrated using as heating medium vapor separated from condensate discharged from the vapor compression evaporator.

Finnish patent publication 118131 (EP 839949) discloses an arrangement for increasing the capacity of a black liquor multi-effect evaporation plant so that the pressure of dis- charge vapor from at least one evaporation effect is increased by means of a compressor or a fan. The vapor at the higher pressure is led into the following evaporation effect as heating medium, so that the question is not about so-called vapor compression evaporation.

As the prices of additional fuel and green electricity - which term generally refers to electricity produced with other than fossil fuel - are continuously increasing, various plant solutions which decrease the steam consumption of the mill are becoming more and more cost-effective. As steam consumption is decreased, the mill balance either saves in additional fuel or more electricity will be available to the condensate part of a turbine for elec- tricity production. Pulp mills are tending to sell out more electricity due to increased eco-

nomic interest. In addition to chemical pulp mills, also other production plants, such as paper mills, aim at more efficient and more economic energy consumption.

The energy consumption of a mechanical vapor compression pre-evaporation plant, which can also be called a fan pre-evaporation plant, if a fan is used, compared to e.g. a 7-effect in-series connected evaporation plant, when calculated in terms of money, is often as much as 70 % lower. Earlier fan pre-evaporation plants have been used at a sulfate pulp mill mainly for obtaining additional capacity for the evaporation plant. Only a portion of the required total evaporation can be carried out at the fan pre-evaporation plant, because excessively high dry solids content of the liquor to be evaporated increases the boiling point rise and requires a high pressure increase from the fan or compressor, which increases energy consumption.

An object of the present invention is provide a method of coupling vapor compression evaporation to an arrangement for evaporating liquids, such as to a multi-effect black liquor evaporation plant known per se in a way that is more advantageous both in view of equipment arrangement and in view of economical aspects.

In order to achieve these goals the present invention relates to an arrangement for evaporating a liquid, such as black liquor, comprising at least a vapor compression evaporation plant including at least one heat transfer surface unit and a vapor pressure increasing device, and a multi-effect in-series connected evaporation plant, each evaporation effect of which comprises at least one heat transfer surface unit and which has conduits for leading a liquid from one evaporation heat transfer unit into another unit and conduits for feeding steam into each of the units and for discharging therefrom the vapor generated in the evaporation of the liquid. The arrangement according to the invention is characterized in that a unit of the vapor compression evaporation plant and one unit of the multi-effect in-series connected evaporation plant are connected so that vapors generated in the units in the evaporation of the liquid are led into a common space that is connected to at least the device for increasing vapor pressure for leading a portion of the vapor therein and for returning it after the pressure increase into the unit of the vapor compression evaporation plant as heating medium. Preferably the common space is further connected to a conduit for leading a portion of the vapors into a next evaporation effect of the multi-effect in-series connected evaporation plant.

The arrangement according to the invention can be applied in connection with multi- effect evaporation plants for different liquids at various production plants. Preferably the invention can be applied in the evaporation of liquids generated or produced in pulp and paper mills. These kinds of liquids in a chemical pulp mill comprise in addition to black liquor also e.g. washing and bleaching filtrates of pulp, as well as liquids originating from effluent treatment. Also other liquids, in addition to liquids generated in sulfate pulp production, can be evaporized in accordance with the invention. These include TMP (ther- momechanical pulping), CTMP (chemical thermomechanical pulping) and APMP (alkaline peroxide thermomechanical pulping) -processes. Additionally, for replacing fossil energy sources, mills can be provided with production plants for other energy sources, such as biodiesel and ethanol, the production of which produces various liquids, in the treatment of which evaporation can be included as one partial process.

The invention is implemented in accordance with one preferred embodiment so that a unit of the vapor compression evaporation plant and one evaporation unit of the multi- effect in-series connected evaporation plant are arranged in a common housing-vessel, i.e. in one and the same evaporation device. Thus, a single evaporation device has two heat transfer surface units, one of which uses as heating medium vapor that is generated in a preceding effect of the multi-effect evaporation plant and the other uses vapor coming from the vapor pressure increasing device. As the liquid is evaporated on an outer surface of heat transfer surface units, vapor generated in the evaporation enters the same space defined by the housing and is discharged typically via a conduit or conduits positioned in the housing, typically at the upper part thereof. A portion of this vapor is used in a following evaporation effect being at a lower pressure and the pressure of a portion of the vapor is increased and returned as heating medium into the unit of the vapor compression evaporation plant.

A falling film evaporation device can preferably be utilized as a heat transfer surface unit, where the evaporation surface is formed of a number of lamellas or tubes. Preferably the liquid to be evaporated , such as black liquor, flows on the outer surface of the lamellas or on the inner surface of the tubes, while the heating steam is led inside the lamellas or to the outer side of the tubes. For instance in plate-type falling film lamella evaporators almost all or all the vertical part of the evaporation surface is in contact with the surrounding vapor space, whereby free space is left between the heat transfer elements for releas- ing the vapor generated on their surface. In an evaporator operating on the freely falling

film principle the vapor flows freely in a space defined by a housing-vessel, inside which vessel the heat transfer element units are located. The generated vapor flows essentially vertically upwards in the free space between the heat transfer element units and the wall of the housing vessel to the upper part of the vessel. Normally the vapor is collected into a drop separator and it is allowed to flow out from the upper part of the vessel. Each of the effects of a multi-effect in-series connected evaporation plant comprises one heat transfer surface unit arranged inside the housing or several parallel connected heat transfer surface units each arranged inside a housing.

A fan or a compressor acts as the steam pressure increasing device.

In accordance with with an embodiment of the invention, a unit of the vapor compression evaporation plant and one unit of the multi-effect in-series connected evaporation plant are arranged inside a common housing, i.e. in a common evaporation device. Thus, a single device has two heat transfer surface units, one of which uses as heating medium vapor that is generated in a preceding effect of the multi-effect in-series connected evaporation plant and the other uses vapor coming from the vapor pressure increasing device. In the units the secondary vapors generated in the evaporation of the liquid are led into one common space, which is a secondary vapor space of the evaporation device and wherefrom they are according to an embodiment discharged together via a common conduit. This discharge conduit is connected to an evaporation effect following in the steam flow direction, wherein a portion of the vapor is introduced directly as heating medium. Said discharge conduit is further connected to a pressure increasing device for increasing the vapor pressure and for returning a portion of the vapors after the pressure increase into the unit of the vapor compression evaporation plant as heating medium.

According to another embodiment of the invention a unit of the vapor compression evaporation plant and one unit of the multi-effect in-series connected evaporation plant are arranged inside a common housing so that a common space for secondary vapor is partly divided by means of an intermediate wall in such a way the wall is provided with an opening or openings for equalizing the pressure between the parts formed by the intermediate wall inside the housing. A purpose of dividing the common vapor space into a partly combined vapor space is to boost the separation of foul condensate. It is known that when a liquid, such as feed liquor or weak black liquor is evaporated the first time, a great amount of substances (e.g. methanol), the boiling point of which is lower than that

of water, boils off first. When these vapors, together with water vapor are led via a compressor or a fan as heating medium into the bottom part of an evaporation heat surface unit of the same effect or in an in-series connected evaporation plant into the bottom part of a following effect's heating steam side, and when the mixture flows upwards, mainly water is condensed therefrom first and when approaching the upper part, whereby the vapor pressure decreases, more and more other vapors are condensed. Easily volatile substances (such as methanol) are accumulated into condensate that is collected separately from the final part of the heat transfer surface (a so-called segregation part for condensate) and this condensate is called foul condensate.

According to an embodiment of the invention, said unit of the vapor compression evaporation plant and one unit of the multi-effect in-series connected evaporation plant are arranged inside separate housing vessels. The discharge of secondary vapors formed therein from the housings is arranged in a common space that is connected to a vapor pressure increasing device for returning a portion of the vapors to the unit of the vapor compression evaporation plant as heating medium. Preferably the space is further connected to a conduit for leading a portion of the vapors into a next evaporation effect of the multi-effect in-series connected evaporation plant. According to one method, both housings are provided with a discharge conduit, which conduits outside the housings merge into one space that is connected both to the vapor pressure increasing device and to a following effect of the multi-effect in-series connected evaporation plant. The coupling of the discharge conduits can also be effected so that the vapor discharge conduit of one housing vessel is connected to the vapor space of the other housing vessel which vapor space acts as a common space and wherefrom secondary vapors are taken partly into the vapor pressure increasing device for returning a portion of the vapors into the unit of the vapor compression evaporation plant as heating medium and partly directly into another effect of the multi-effect in-series connected evaporation plant as heating medium.

According to a preferred embodiment of the invention, the discharge, i.e. venting of non- condensable gases generated in a unit of the vapor compression evaporation plant is effected by means of the respective devices of the multi-effect evaporation plant.

According to a preferred embodiment of the invention, the discharge and treatment of condensates generated in a unit of the vapor compression evaporation plant are handled with dedicated devices of the multi-effect evaporation plant.

The invention is described in more detail with reference to the accompanying figures, of which

Fig. 1 illustrates schematically in detail an arrangement of a unit of a vapor compression evaporation plant in connection with a preferred embodiment of the present invention, Fig. 2 illustrates schematically a coupling of a vapor compression evaporation plant and a multi-effect evaporation plant in accordance with the invention, and

Fig. 3 illustrated schematically another preferred arrangement in accordance with the invention.

The invention is described in more detail by applying it to black liquor evaporation. Fig. 1 illustrates an evaporation device with a housing 2, inside which a heat transfer surface unit 4 of a vapor compression evaporation plant and a heat transfer surface unit 3 of a multi-effect evaporation plant are arranged, both operating on the falling film principle. In this embodiment the heat transfer surface units are formed of a number of lamellas, to the interior of which heating steam is fed, and the liquor to be evaporated flows on the outer surface thereof. Thereby, liquor is thus heated by means of indirect contact with the steam inside the heat transfer surface units. Instead of lamellas, also tubes can be used, whereby the liquor can flow on the inner or outer surface thereof.

The black liquor, feed liquor or liquor from an earlier evaporation effect, which liquor is to be evaporated, is fed via pipe 5 into the lower part of the evaporation device, wherefrom the liquor is pumped by means of a pump 6 via pipe 7 into a distribution device 19 located above the heat transfer units. In the distribution device the liquor flows via openings or corresponding to the outer surface of the lamellas, where liquor is evaporated. Liquor to be evaporated is discharged into a following evaporation effect via line 13. A heat transfer unit of the multi-effect evaporation plant receives as heating medium typically vapor via pipe 8 from a preceding evaporation effect operating at a higher pressure. The heat transfer surface unit 4 of the vapor compression evaporation plant receives as heating medium steam from compressor 9 via line 12.

Because the heat transfer surface unit 4 of the vapor compression evaporation plant and the heat transfer surface unit 3 of the multi-effect evaporation plant are located in a common housing 2, vapors evaporated from black liquor on their outer surface are led into a common space, which is this embodiment is formed of the upper part 18 of the housing vessel. A conduit or channel 10 is connected thereto. The pipe 10 is provided with a vapor pressure increasing device 9, which typically is a compressor or a fan. In the pressure increasing device 9, the vapor pressure is increased to a level at which it can be returned as heating medium back to the same effect, into the vapor compression heat transfer surface unit 4. A branch pipe 11 is also connected to pipe 10, via which branch pipe a portion of the vapor discharged from evaporator 1 is taken as heating medium into another evaporation effect.

In heat transfer surface units 3 and 4 the heating steams form condensate, which is discharged via pipes 15 and 16 into a expansion tank and therefrom further via line 17 into condensate treatment of the multi-effect evaporation plant. The non-condensable gases generated in said units are also discharged via a common line 14 to further treatment.

An advantage of the present invention is better possibility to utilize apparatuses, such as transfer pipings for liquor and steam, condensate system and vacuum system of a multi- effect evaporation plant, compared to known couplings.

Figure 2 illustrates a 7-effect multi-effect in-series connected black liquor evaporation plant. In this case the evaporation plant comprises sequential effects I-VII, which operate at pressures and temperatures that decrease sequentially in the flow direction of the steam. The final evaporation effect I of the liquor comprises steps IA and IB. The evaporators illustrated in Figure 2 are falling film lamella evaporators, but other evaporators suitable for black liquor evaporation can be used as well in this case. Figure 2 uses the same reference numerals as figure 1 where applicable.

For the evaporation, fresh steam of the mill is typically fed via channel 30 into steps IA and IB of evaporation effect I so that it warms the black liquor and simultaneously condenses. In evaporation effect I vapor separates from black liquor, which vapor is taken as heating medium into effect Il via channel 31. In evaporation effect II, vapor separates that is at a lower temperature than in evaporation effect I, which vapor is further led into a following evaporation effect III via channel 32. Accordingly, in evaporation effects III, IV, V and Vl the

secondary vapor separated from black liquor is taken to a respective following evaporation effect IV, V, Vl and VII, for warming and evaporating black liquor.

The weak liquor (feed liquor) is introduced via line 5 ^' into effect IV, from where it flows via line 13 into effect VII. Liquor from effect VII is introduced for evaporation into effect Vl via line 40 and further into effect V via line 41 for formation of intermediate liquor in line 42. The intermediate liquor is further led to effect III, wherefrom the liquor is taken via evaporation effect Il to final evaporation I provided with two steps IA and IB connected in series at the liquor side. Liquor is first evaporated in step IB, from where it is led to step IA for evaporating the liquor to a high dry-solids content, i.e. approximately 75-90%. The concentrated combustion liquor is discharged via line 43 to combustion.

In the last effect VII the produced vapor is taken via line 33 to the vacuum system of the evaporation plant, where it is e.g. cooled by means of cooling water in a surface con- denser (vacuum condenser) (not shown).

In accordance with the invention, a vapor compression evaporation plant is connected in the first evaporation effect , in relation to the black liquor flow direction, of a multi-effect in- series connected evaporation plant. The connection is similar to that presented in more detail in Figure 1. Effect IV is provided with a heat transfer surface unit 4 ^' of a vapor compression evaporation plant and a heat transfer surface unit 3 ^' of a multi-effect evaporation plant, which operate on the falling film principle and are located inside a common housing. The feed liquor 5 ^' is introduced into the bottom part of the evaporation device of evaporation effect IV, whereform the liquor is pumped via a distribution device to the outer surface of heat transfer surface units 3 ^' and 4 ^' , whereby as the liquor flows downwards on the surface, vapor is evaporated therefrom. The secondary vapor formed in the evaporation is discharged from the upper part of the evaporator via conduit 10. A portion of the vapor is taken to the next evaporation effect V in the steam flow direction. A portion of the vapor is again led into a vapor pressure increasing device 9 arranged in con- duit 10, wherein the pressure of the vapor is increased so that this vapor can be returned as heating medium into heat transfer surface unit 4 ^' of the vapor compression evaporation plant. The condensates 17 and 23 generated in the heat transfer units can be treated in the condensate treatment system of the multi-effect evaporation plant.

Figure 3 illustrates an alternative construction for the embodiment of Figure 1. In Figure 3 - which uses the same reference numerals as Figs. 1 and 2 where applicable - the heat transfer surface unit 4 of a vapor compression evaporation plant and the heat transfer surface unit 3 of a multi-effect evaporation plant are separated by a light intermediate wall 20. The secondary vapor formed in the units enters a common space, because the intermediate wall is provided with an opening 21 or openings for equalizing the pressure on both sides of the intermediate wall 20. This construction boosts the separation of foul condensate.

The weak feed liquor is introduced via line 5 into the evaporation unit to the bottom of housing vessel 2, wherefrom it is pumped by means of a pump 6' via line T into a distribution device located above the vapor compression heat transfer surface unit 4. The liquor flows downwards on the outer surface of the heat transfer unit 4, and the concentrated liquor is discharged via line 25 into a suitable evaporation effect. Generated sec- ondary vapor is led via conduit 10 ^" located in the upper part of the housing to a pressure increasing device 9. Vapor at an increased pressure is led via line 12 into the lower part of the interior of heat transfer unit 4, where it flows upwards. As the vapor flows upwards, water is mainly evaporated first and when approaching the upper part and when the pressure of the water vapor decreases, more and more other vapors are evaporated. Easily volatile substances (such as methanol) are accumulated into condensate that is collected separately from the final part of the heat transfer surface (a so-called segregation part for condensate), which final part is separated by a short intermediate wall 22. This condensate portion is called foul condensate, which is discharged via line 23.

Liquor is introduced into the multi-effect evaporation plant's heat transfer unit 3 located in housing vessel 2 from the second evaporation effect via line 26. The liquor is taken by means of a pump 6 ^" via line 7 ^" into the upper part of said unit and made to flow downwards on its outer surface. Evaporated secondary vapor is discharged from housing 2 via conduit 10' and taken in the steam flow direction into the next evaporation effect. Segre- gation of condensate can be effected also in this unit in the same way as described above. The clean condensate is discharged from the lower part of the interior of the heat transfer units via lines 15 and 16 further via line 17. Non-condensable gases are discharged via line 14. An advantage of the invention is that the treatment of condensates and the treatment of non-condensable gases can be effected with the same devices, i.e.

devices of the multi-effect evaporation plant, and the vapor compression evaporation plant does not need its own separate devices.

By connecting the black liquor vapor compression pre-evaporation plant to the multi-effect evaporation plant in accordance with the invention, at least the following advantages can be gained:

- the number of equipment, pipings etc. required by the vapor compression evaporation plant decreased, because it is possible to better utilize common systems, such as transfer pipings for liquor and vapor, the condensate system and the vacuum system. - steam consumption is decreased compared to a multi-effect evaporation plant, whereby more steam can be used for e.g. the production of electric energy -as to economy, the connection of the vapor compression evaporation plant and the 7- effect in-series connected evaporation plant illustrated in Figure 2 corresponds to a 9-11 -effect in-series connected evaporation plant depending on the balance of the mill and the price of additional fuel and electricity. Thus, remarkable savings are obtained also, as in according with the invention, the number of equipment required is smaller. The same advantages are reached also in connection with evaporation of other liquids.

While the invention has herein been illustrated and described in connection with what at present is considered to be the most practical and most preferred embodiment, it is obvious to persons skilled in the art that many modifications can be made within the scope of the invention, which scope is to be given the broadest possible interpretation in accordance with the appended claims in order to cover all corresponding systems and processes.