System and method for concentrating substance-containing fluid by means of multi-stage evaporation

Abstract

The invention relates to a system (1) for concentrating a substance-containing fluid by means of multi-stage evaporation, in particular a solution such as N-methylmorpholine-N-oxide (NMMO), in which the system (1) is provided with at least a first evaporator (2) and a second evaporator (3), wherein the first evaporator (2) is suitably connected to the second evaporator (3) such that the fluid concentrated in the first evaporator (2) can be conveyed into the second evaporator (3) for further concentration of the concentrated fluid in the second evaporator (3), and wherein a first mechanically operated compressor unit (4) is provided, with which the first mechanically operated compressor unit (4) is provided, the steam formed in the first evaporator (2) may be compressed downstream, and wherein the system (1) comprises a first supply line (6) for supplying the steam compressed by the first mechanically acting compressor unit (4) into the first evaporator (2). According to the invention, a second mechanically active compressor unit (5) is provided, by means of which steam formed in the second evaporator (3) can be compressed downstream, the steam compressed by the second compressor unit (5) can be supplied to the first compressor unit (4). The invention also relates to a method for concentrating a substance-containing fluid by means of multi-stage evaporation.

Description

Technical Field

[0001] The invention relates to a system for concentrating a fluid containing a substance by multi-stage evaporation, in particular multi-stage evaporation of a solution such as N-methylmorpholine-N-oxide (NMMO), wherein the system is configured with at least a first evaporator and a second evaporator, wherein the first evaporator is connected to the second evaporator in a suitable manner so that the fluid concentrated in the first evaporator can be conducted to the second evaporator in order to further concentrate the concentrated fluid in the second evaporator, wherein a first mechanical action compressor unit is provided, with which the vapor formed in the first evaporator can be compressed downstream, and wherein the system comprises a first supply line for supplying the vapor compressed by the mechanical action compressor unit to the first evaporator.

[0002] The invention further relates to a method for concentrating a fluid containing a substance by multi-stage evaporation, for example multi-stage evaporation of a solution such as NMMO, in particular with an apparatus of the type defined above, wherein the fluid is concentrated in a first evaporator and the fluid concentrated in this way is conducted to a second evaporator, in which the concentrated fluid is further concentrated, after which the concentrate is removed, wherein the vapor formed in the first evaporator is compressed downstream with a first mechanically acting compressor unit and is at least partially supplied to the first evaporator via a first supply line. Background Art

[0003] In the context of the present invention, the term "fluid containing substances" should be understood to mean fluids that, in addition to the fluid, also have substances dissolved therein, thereby entrained or present therein in another form, such as salt-containing fluids, fluids with dissolved polar organic molecules or oligomers, and polymers or other fluids that can be concentrated relative to the substances contained.

[0004] NMMO or other fluids such as ionic fluids are used as solvents, especially for dissolving cellulose for subsequent applications. Lyocell fibers, films or other products can be generated, for example, from suitable solutions. In particular, NMMO has the advantages of being non-toxic and having no strong odor, and is therefore used preferentially. For this reason, and for environmental protection reasons, it is also desirable to recycle the solvent used for the process, so that it can be supplied to the process in a state as pure as possible. At this point, it is necessary to concentrate the appropriate solvent in a manner that allows the appropriate solution to be used again for the corresponding process.

[0005] In order to be able to supply NMMO again to the process for producing lyocell fibers, for example, US 2011 / 0226427 A1 shows that it is known to reuse the steam obtained by concentrating NMMO in solution in order to be able to operate in a manner that is as energy-efficient as possible. In this regard, the steam discharged from the evaporator is compressed, and in fact compressed by means of mechanical compression.

[0006] Generally, in order to concentrate the solution containing substances, it is known to use a multi-stage method utilizing multiple evaporators. As an example, pre-concentration can be performed in the first evaporator, and further concentration is performed in the second evaporator thereafter, and the second evaporator is preferably equivalent to the final evaporator. Suitable examples are provided in DE 10 2012 203 439 A1. Here, as a rule, the first evaporator is operated in the following manner, that is, the steam escaping from the evaporator is made to reach a higher temperature by means of mechanical compression, and then is resupplied to the evaporator so as to be able to operate in a manner that saves more energy as much as possible. The second evaporator should also be operated in an energy-efficient manner as much as possible. However, the concentration in the first evaporator often causes the boiling point of the concentrated solution to be much higher than the solution with a much smaller concentration present in the first evaporator. Therefore, in order to operate the second evaporator in a manner that enables the desired concentration to be performed in the second step (usually the last step), the increased energy consumption is necessary. Therefore, as disclosed in the cited document DE 10 2012 203 439 A1, it is necessary to operate the second evaporator with separately generated live steam or to operate the second evaporator to provide thermal compression, both of which consume much more energy than mechanical compression. Summary of the invention

[0007] This is the starting point of the present invention. The object of the present invention is to further develop a device of the aforementioned type in such a way that it can be operated more energy efficiently, in particular in such a way that it is possible to operate completely or partially without thermal compression.

[0008] A further object of the present invention is to further develop a method of the aforementioned type in such a way that it can be carried out in an energy-efficient manner, in particular without having to resort to thermal compression.

[0009] The objects of the invention are achieved when in an arrangement of the aforementioned type a second mechanically acting compressor unit is provided, with which the steam formed in the second evaporator can be compressed downstream, wherein the steam compressed by the second compressor unit can be supplied to the first compressor unit.

[0010] The advantage of the concept of the invention is to configure the system in such a way that concentrated utilization of substance-containing fluids, in particular NMMO-containing fluids, but also ionic fluids is possible with respect to simple means. This means that in particular NMMO solutions can be concentrated effectively and in an energy-efficient manner and can therefore, for example, be regenerated in a process for the production of lyocell fibers and then supplied to the process.

[0011] Although not mandatory, the device according to the invention can be operated without a thermocompressor. In particular, it is not absolutely necessary to supply fresh steam to the second evaporator, in which, as a rule, the final concentration takes place. For this purpose, the system is designed so that the steam produced in the second evaporator is compressed by means of a second mechanical action compressor unit and thereafter is supplied in full or at least in part to the first mechanical action compressor unit, so that a further compression of the already compressed steam from the second evaporator is carried out. The steam that has been compressed in this way is then supplied to the first evaporator and the second evaporator, wherein the finally compressed steam is divided into suitable partial flows.

[0012] The following considerations form the basis for the design of the system according to the invention: NMMO solutions and the like constitute valuable resources in the production process, which should therefore be recycled. In processes such as the Lyocell method, for example, such solutions appear with low NMMO contents of, for example, about 20% (as a percentage by weight, hereinafter abbreviated as % by weight). In order to be able to supply such solutions to the actual production process again, the solution needs to be concentrated to, for example, about 80% by weight. However, the concentration cannot be carried out in an energy-efficient manner in a single evaporator, but must be carried out in technically continuous compression steps until the desired final concentration of NMMO is reached. The problem here is that the increase in boiling point is considerable as the concentration of NMMO in the fluid increases. If, for example, a two-stage process is operated using two evaporators, the boiling temperature in the first evaporator may be about 120°C, while in the second evaporator (which has a much higher concentration of NMMO), the boiling temperature may be about 145°C. This means that a significantly increased amount of energy must be supplied in the second evaporator in order to extract steam and thus achieve concentration. Although according to the prior art, in the first evaporator, steam can be extracted, compressed and supplied again to the first evaporator, this is not possible for the second evaporator, since the steam escapes at a relatively low temperature of about 85° C. to 90° C., and any mechanical compression of the steam and the corresponding temperature increase are not sufficient to be able to supply sufficient energy to the second evaporator for concentration. Therefore, according to the prior art, powerful thermocompressors are provided for the second evaporator, or alternatively, they are additionally operated with a separate supply of fresh steam or are operated exclusively with a separate supply of fresh steam. In contrast, according to the invention, a second mechanical compressor unit is now provided for the steam from the second compressor, wherein the compressed steam from the second mechanical compressor unit is first supplied to the first compressor unit, and then the steam that has finally been compressed in this way is divided into partial flows and supplied to the first evaporator on the one hand and to the second evaporator on the other hand. This measure means that the steam extracted from the second evaporator is not only compressed by the second compressor unit, but also by the first compressor unit. By suitable configuration, the second compressor unit can provide a temperature increase of about 30° C. for the steam, which is not enough for the second evaporator. However, by supplying the steam compressed by the second evaporator to the first compressor unit, a further temperature jump upward can be obtained so that the temperature is sufficient for the operation of the second evaporator and the steam brought to temperature in this way can also be supplied to the second evaporator at a suitable temperature. With the help of a suitable circuit system, the energy consumption for the operation of the two evaporators and therefore of the system as a whole is minimized.

[0013] It may be the case that the first compressor unit comprises a plurality of compressors. Suitably, the individual plurality of compressors are connected in series. Preferably, the first compressor unit has two to four, in particular two or three compressors, in order to obtain a desired temperature increase for the steam taken from the first evaporator and for the already compressed steam supplied from the second evaporator. Furthermore, if desired, a parallel connection of a plurality of individual compressor units can be envisaged, which in turn can comprise a plurality of compressors, wherein all or part of the steam from the second compressor unit is supplied to at least one of these compressors.

[0014] The second compressor unit can also comprise a plurality of compressors. The number of compressors in the second compressor unit in turn adapts to the design of the system. In principle, for the second compressor unit, in a manner similar to the first compressor unit, it is sufficient to have two or three compressors connected in series. In both compressor units, the compressors can be configured as known vapor recompressors. Furthermore, here again, if necessary, a parallel connection of a plurality of individual compressor units can be envisaged, which in turn can each comprise a plurality of compressors.

[0015] The compressed steam from the first mechanical action compressor unit (and also the further compressed steam from the second mechanical action compressor unit) is then reused in the process to heat the first evaporator and the second evaporator. To this end, the system comprises a second supply line for supplying the steam compressed by the first mechanical action compressor unit to the second evaporator.

[0016] In principle, with the system according to the invention, a thermocompressor is not provided. Nevertheless, in certain cases it may be necessary to additionally provide a thermocompressor of small dimensions for the second evaporator, but this should in principle be avoided.

[0017] The second evaporator is preferably configured as a final evaporator from which the concentrated substance-containing fluid is discharged from the system. Although the system may have more than two evaporators, for most purposes it is sufficient to configure the system with a first evaporator for pre-concentration and a second evaporator as a final evaporator for achieving final concentration.

[0018] The objects associated with the method are achieved when, in a method of the aforementioned type, the vapor formed in the second evaporator is compressed downstream with a second mechanical action compressor unit and then supplied to the first compressor unit.

[0019] According to the concept of the invention, it is thus possible for an evaporator to be provided with a desired temperature with minimized energy consumption. This is possible because the steam extracted from the second evaporator is first compressed with the second mechanical action compressor unit, has a first temperature increase, and is then further compressed with the first mechanical action compressor unit and reaches a higher temperature, so that after the final compression, a suitable partial flow can be formed for heating the evaporator.

[0020] The steam compressed in the second evaporator can be compressed in the first mechanical action compressor together with the steam formed in the first compressor, so that at the end of the first mechanical action compressor unit, a combined steam flow is produced from the two steam portions extracted from the first evaporator and from the second evaporator, which combined steam flow can then be divided into a first partial flow for the first evaporator and a second partial flow for the second evaporator.

[0021] The steam formed in the first evaporator can be compressed downstream using a plurality of compressors of the first mechanical action compressor unit. In addition, with respect to the second compressor unit, a plurality of compressors can be provided. The number of compressors is designed so that, on the one hand, the desired temperature rise is obtained for resupplying the steam to the evaporator, but on the other hand, the energy consumption is minimized.

[0022] In the context of the present invention, the steam in the second compressor unit can be compressed to a higher outlet temperature minus the inlet temperature than in the first compressor unit. As a rule, it is necessary to subject the steam escaping from the second evaporator to a higher temperature increase, because in the second evaporator not only a higher energy supply is necessary, but also the steam is discharged at a lower temperature.

[0023] In the context of the present invention, as a rule, the first evaporator is operated at a higher pressure than the second evaporator.

[0024] Further features, advantages and effects of the present invention will become apparent from the exemplary embodiments described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In the accompanying drawings, Figure 1 A system design according to the present invention is shown. DETAILED DESCRIPTION

[0026] Figure 1A system 1 designed for concentrating NMMO is shown. The system 1 comprises a first evaporator 2 and a second evaporator 3. An aqueous solution of NMMO is introduced into the evaporator circuit for the purpose of concentrating the solution. The NMMO solution is supplied to the first evaporator 2 for pre-concentration with, for example, a NMMO content of 20% by weight. In the first evaporator 2, the temperature of the supplied NMMO solution is increased by supplying energy. The steam thus produced can be discharged overhead in the first evaporator 2. The pre-concentrated NMMO solution is taken out from the bottom and supplied to the second evaporator 3, in which, in turn, the temperature of the supplied solution is increased so that, in turn, steam can be extracted from the top of the second evaporator 3, and so that, in turn, an increase in concentration occurs, and finally, a NMMO solution with 80% by weight of NMMO can be discharged from the bottom of the second evaporator 3, which serves as a final evaporator.

[0027] The steam escaping from the first evaporator 2 is reused in a further process. For this purpose, a first mechanically acting compressor unit 4 is provided. The first compressor unit 4 comprises a plurality of individual compressors 41. In the exemplary embodiment, the first compressor unit 4 comprises two compressors 41. The two compressors 41 of the first compressor unit 4, which are connected in series, are connected to the first evaporator 2 via a first discharge line 42. In a similar manner, the second evaporator 3 is designed with a discharge line 52, which connects the top outlet of the second evaporator 3 to the second mechanically acting compressor unit 4. The second compressor unit 5 also comprises two individual compressors 51. The compressors 51 of the second compressor unit 5 are also connected in series.

[0028] exist Figure 1 As can be seen in FIG. 5 , the compressor 51 of the second compressor unit 5 is equipped with a downstream line which discharges into the discharge line 42 of the first evaporator 2. The steam compressed with the second compressor unit 5 is thus combined with the steam discharged from the first evaporator 2, and the steam combined in this way is then further compressed in the first compressor unit 4. After this final compression, the compressed steam is divided into two partial flows, namely a first partial flow for the first supply line 6 of the first evaporator 2, and a second supply line 7 for the second evaporator 3.

[0029] By means of the provided circuit, it is possible to bring the steam from the second evaporator 3 (escaping at a relatively low temperature of about 30°C lower than the temperature in the first evaporator 2) to a higher temperature of about 30°C to 40°C first with the provided compressor 51 of the second compressor unit 5. This alone would not be a sufficient temperature for the operation of the second evaporator 3. However, by additional compression using the first compressor unit 4, a further temperature increase of several degrees Celsius is obtained so that the temperature of the compressed steam is sufficient to sufficiently heat the second evaporator 3. Thus, a minimum number of mechanically acting compressors is sufficient to result in an energy-efficient process. Figure 1 As can be seen in FIG. 1 , in the first evaporator 3 the NMMO solution can thus be concentrated to about 40 wt. % NMMO and in the second evaporator to a final 80 wt. % NMMO.

Claims

1. A system (1) for concentrating a fluid containing a substance by multi-stage evaporation, in particular multi-stage evaporation of a solution such as N-methylmorpholine-N-oxide (NMMO), wherein the system (1) is configured with at least a first evaporator (2) and a second evaporator (3), wherein the first evaporator (2) is connected to the second evaporator (3) in a suitable manner so that the fluid concentrated in the first evaporator (2) can be conducted to the second evaporator (3) in order to further concentrate the concentrated fluid in the second evaporator (3), wherein a first mechanical action compressor unit (4) is provided, with which the steam formed in the first evaporator (2) can be compressed downstream, and wherein the system (1) comprises a first supply line (6) for supplying the steam compressed by the first mechanical action compressor unit (4) to the first evaporator (2), characterized in that A second mechanically-acting compressor unit (5) is provided, with the aid of which the steam formed in the second evaporator (3) can be compressed downstream, wherein the steam compressed by the second compressor unit (5) can be supplied to the first compressor unit (4).

2. The system (1) according to claim 1, characterized in that The first compressor unit (4) comprises a plurality of compressors (41).

3. The system (1) according to claim 1 or claim 2, characterized in that The second compressor unit (5) comprises a plurality of compressors (51).

4. The system (1) according to any one of claims 1 to 3, characterized in that The system (1) comprises a second supply line (6) for supplying steam compressed by the first mechanical action compressor unit (4) to the second evaporator (3).

5. System (1) according to any one of claims 1 to 4, characterized in that A thermal compressor is not provided.

6. System (1) according to any one of claims 1 to 5, characterized in that The second evaporator (3) is configured as a final evaporator from which the concentrated fluid is discharged from the system (1).

7. A method for concentrating a fluid containing a substance by multi-stage evaporation, for example of a solution such as N-methylmorpholine-N-oxide (NMMO), in particular with a device (1) according to any one of claims 1 to 6, wherein the fluid is concentrated in a first evaporator (2) and the fluid concentrated in this way is conducted to a second evaporator (3) in which the concentrated fluid is further concentrated, after which the concentrate is removed, wherein the vapor formed in the first evaporator (2) is compressed downstream with a first mechanically acting compressor unit (4) and is at least partially supplied to the first evaporator (2) via a first supply line (6), characterized in that The steam formed in the second evaporator (3) is compressed downstream with a second mechanical action compressor unit (5) and then supplied to the first compressor unit (4).

8. The method according to claim 7, characterized in that The steam compressed in the second evaporator (3) is compressed in the first mechanical action compressor unit (4) together with the steam formed in the first evaporator (2).

9. The method according to claim 7 or claim 8, characterized in that The steam compressed in the first mechanically acting compressor unit (4) is divided into partial flows, one of which is supplied to the first evaporator (2) and a further partial flow is supplied to the second evaporator (3).

10. The method according to any one of claims 7 to 9, characterized in that The steam formed in the second evaporator (3) is compressed downstream by means of a plurality of compressors (51) of the second mechanical action compressor unit (5).

11. The method according to any one of claims 7 to 10, characterized in that The steam formed in the first evaporator (2) is compressed downstream by means of a plurality of compressors (41) of the first mechanical action compressor unit (41).

12. The method according to any one of claims 7 to 11, characterized in that The first evaporator (2) is operated at a higher pressure than the second evaporator (3).

13. The method according to any one of claims 7 to 12, characterized in that The steam in the second compressor unit (5) is compressed to a higher outlet temperature than in the first compressor unit (4) minus the inlet temperature difference.

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