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

Abstract

The invention relates to a system (1) for concentrating substance-containing fluids by means of multi-stage evaporation, in particular solutions such as n-methylmorpholine n-oxide (NMMO), wherein the system (1) is provided with at least one first evaporator (2) and a second evaporator (3), wherein the first evaporator (2) is suitably connected to the second evaporator (3) such that fluid concentrated in the first evaporator (2) can be transferred into the second evaporator (3) in order to further concentrate concentrated fluid in the second evaporator (3), wherein a first mechanically operating compressor unit (4) is provided, with which vapour formed in the first evaporator (2) can be compressed downstream, and wherein the system (1) comprises a first supply line (6) for supplying vapour compressed by the first mechanically operating compressor unit (4) into the first evaporator (2). According to the invention, a second mechanically operating compressor unit (5) is provided, with which vapour formed in the second evaporator (3) can be compressed downstream, wherein vapour 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 substance-containing fluids by means of multi-stage evaporation.

Description

[0001] Plant and process for the concentration of substance-containing liquids by multi-stage evaporation

[0002] The invention relates to a plant for concentrating substance-containing liquids by multi-stage evaporation, in particular of solutions such as N-methylmorpholine-N-oxide (NMMO), wherein the plant is designed with at least a first evaporator and a second evaporator, wherein the first evaporator is suitably connected to the second evaporator so that liquid concentrated in the first evaporator can be fed into the second evaporator in order to further concentrate concentrated liquid in the second evaporator, wherein a first mechanically operating compressor unit is provided, with which vapor formed in the first evaporator can be compressed downstream, and wherein the plant comprises a first supply line for supplying vapor compressed by the first mechanically operating compressor unit to the first evaporator.

[0003] Furthermore, the invention relates to a method for concentrating substance-containing liquids by multi-stage evaporation, for example of solutions such as NMMO, in particular with a device of the type mentioned above, wherein the liquid is concentrated in a first evaporator and the thus concentrated liquid is fed to a second evaporator in which the concentrated liquid is further concentrated, after which a concentrate is withdrawn, wherein vapor formed in the first evaporator is compressed downstream with a first mechanically operating compressor unit and is fed at least partially to the first evaporator via a first feed line.

[0004] In the context of the invention, substance-containing liquids are understood to mean liquids which, in addition to the liquid, contain a substance dissolved in it, carried by it or present in another form, for example salt-containing liquids, liquids with dissolved polar organic molecules or oligomers as well as polymers or other liquids which can be concentrated with respect to the substance they contain.

[0005] NMMO or other liquids such as ionic liquids are used as solvents to dissolve cellulose, among other things, for subsequent processing. Lyocell fibers, films, or other products can be produced from such a solution. NMMO, in particular, has the advantage of being neither toxic nor odorous and is therefore preferred. For this reason and for environmental protection reasons, it is also desirable to recycle solvents for the process and thus feed them back into the process in as pure a state as possible. This requires concentrating the corresponding solvents to such an extent that the corresponding solution is usable again for the respective process.

[0006] For example, to recycle NMMO into a process for producing lyocell fibers, US 2011 / 0226427 A1 discloses the reuse of steam obtained by concentrating NMMO in a solution in the process to ensure the most energy-efficient operation possible. Steam extracted from an evaporator is compressed using mechanical compression.

[0007] It is generally known to use multi-stage processes with multiple evaporators for the concentration of substance-containing solutions. For example, a pre-concentration can take place in a first evaporator, followed by further concentration in a second evaporator, preferably immediately a final evaporator. A corresponding example is given in DE 102012 203439 A1. The first evaporator is generally operated in such a way that vapor escaping from the evaporator is brought to a higher temperature by means of mechanical compression and then fed back into the evaporator in order to operate as energy-efficiently as possible. The second evaporator should also be operated as energy-efficiently as possible. However, the concentration in the first evaporator regularly results in the concentrated solution having a significantly higher boiling point than the far less concentrated solution in the first evaporator.This results in increased energy consumption to operate the second evaporator so that the desired concentration can be achieved in the second step, usually a final step. Therefore, as mentioned above,

[0008] As stated in DE 10 2012 203439 A1, it is necessary to operate the second evaporator either with separately generated fresh steam or to provide thermal compression, both of which are significantly more energy-intensive than mechanical compression. This is where the invention comes in. The object of the invention is to further develop a device of the type mentioned above in such a way that it can be operated in an energy-efficient manner, in particular with the option of operating entirely or partially without thermal compression.

[0009] A further aim of the invention is to further develop a method of the type mentioned at the outset in such a way that it can be carried out in an energy-efficient manner, in particular without thermal compression being absolutely necessary.

[0010] The object of the invention is achieved if, in a device of the type mentioned at the outset, a second mechanically operating compressor unit is provided, with which vapor formed in the second evaporator can be compressed downstream, wherein vapor compressed by the second compressor unit can be fed to the first compressor unit.

[0011] One advantage achieved with the invention is that the system is designed in such a way that the concentration of substance-containing liquids, in particular a liquid containing NMMO, but also ionic liquids, is possible with relatively simple means. This allows NMMO solutions in particular to be concentrated effectively and energy-efficiently and thus regenerated, for example, in a lyocell fiber manufacturing process and subsequently fed back into this process.

[0012] Although not mandatory, a device according to the invention can operate without a thermal compressor. In particular, it is also not absolutely necessary to supply fresh steam to the second evaporator, in which final concentration usually takes place. For this purpose, the system is designed such that steam generated in the second evaporator is compressed by a second mechanically operated compressor unit and then fed entirely or at least partially to the first mechanically operated compressor unit, so that the already compressed steam from the second evaporator is further compressed. The thus compressed steam is then fed to the first evaporator and to the second evaporator, with the finally compressed steam being divided into suitable partial streams.The plant concept according to the invention is based on the following ideas: NMMO solutions and the like represent valuable resources in a production process and should therefore be recycled. In processes such as a lyocell process, corresponding solutions with a low NMMO content of, for example, around 20% (in percent by weight, hereinafter referred to as wt.%) arise. In order to be able to feed such solutions back into the actual production process, the solution must be concentrated to, for example, around 80 wt.%. However, such concentration cannot be carried out in an energy-efficient manner in a single evaporator, but must technically be carried out in successive evaporation steps until a desired final concentration of NMMO is reached. The problem here is that with increasing NMMO concentration in the liquid, the boiling point increases considerably.If, for example, a two-stage process is operated with two evaporators, the boiling temperature in the first evaporator can be around 120 °C, whereas in the second evaporator with a significantly higher NMMO concentration the boiling temperature can be around 145 °C. This means that considerably more energy must be supplied to the second evaporator in order to extract vapor and thus achieve concentration. While steam can be extracted from the first evaporator according to the state of the art, compressed and fed back into the first evaporator, this is not possible with the second evaporator because the steam exits at a relatively low temperature of around 85 °C to 90 °C and any mechanical compression and the corresponding temperature increase of the steam is not sufficient to supply the second evaporator with sufficient energy for concentration.Therefore, according to the prior art, powerful thermal compressors are provided for the second evaporator or, alternatively, the second evaporator is operated additionally or exclusively with separately supplied fresh steam. In contrast, the invention provides a second mechanical compressor unit for steam from the second compressor, wherein steam compressed by the second mechanical compressor unit is first fed to the first compressor unit before the finally compressed steam is divided into partial streams and fed to the first evaporator on the one hand and the second evaporator on the other. This measure enables steam drawn off from the second evaporator to be compressed not only by the second compressor unit, but also by the first compressor unit. With a suitable design, the second compressor unit can provide steam with a temperature increase of approximately 30 °C, which is not yet sufficient for the second evaporator.By supplying the steam compressed by the second compressor unit to the first compressor unit, a further temperature increase can be achieved, so that the temperature is sufficient for the second evaporator to operate, and the steam thus heated to the appropriate temperature can also be supplied to the second evaporator at the appropriate temperature. The appropriate circuitry minimizes the energy required to operate both evaporators and thus the system as a whole.

[0013] It can be provided that the first compressor unit comprises several compressors. The individual compressors are expediently connected in series. The first compressor unit preferably has two to four, in particular two or three, compressors in order to achieve the desired temperature increase for the steam taken from the first evaporator and the already compressed steam supplied by the second evaporator. Furthermore, if necessary, a parallel connection of several separate compressor units can also be provided, each of which can in turn comprise several compressors, with steam from the second compressor unit being supplied in whole or in part to at least one of these compressors.

[0014] The second compressor unit can also comprise multiple compressors. The number of compressors in the second compressor unit is again tailored to the system concept. In principle, it is sufficient for the second compressor unit to have two or three compressors connected in series, similar to the first compressor unit. The compressors in both compressor units can be designed as conventional vapor compressors. Furthermore, if required, a parallel connection of several separate compressor units can be provided at this point, each of which can comprise multiple compressors.

[0015] Steam compressed by the first mechanically operating compressor unit, which also includes the further compressed steam from the second mechanically operating compressor unit, is subsequently reused in the process to heat the first evaporator and the second evaporator. For this purpose, the system is provided with a second supply line for supplying steam compressed by the first mechanically operating compressor unit to the second evaporator. In principle, no thermal compressor is provided in the system according to the invention. Nevertheless, it may be necessary under certain circumstances to additionally supply the second evaporator with a small-sized thermal compressor, although this should generally be avoided.

[0016] The second evaporator is preferably designed as a final evaporator, from which the concentrated substance-containing liquid is withdrawn from the system. Although the system may have more than two evaporators, for many purposes it is sufficient if the system is designed with a first evaporator for pre-concentration and a second evaporator as the final evaporator to achieve the final concentration.

[0017] The procedural problem is solved when, in a process of the type mentioned at the outset, vapor formed in the second evaporator is compressed downstream with a second mechanically operating compressor unit and then fed to the first compressor unit.

[0018] According to the concept of the invention, it is thus possible to bring the intended evaporators to the required temperature with minimal energy consumption. This is possible because vapor extracted from the second evaporator is first compressed by the second mechanically operated compressor unit, resulting in an initial temperature increase. It is then further compressed by the first mechanically operated compressor unit and brought to an even higher temperature, so that after the final compression, suitable partial streams can be formed for heating the evaporators.

[0019] The vapor compressed in the second evaporator can be compressed together with the vapor formed in the first evaporator in the first mechanically operated compressor, so that the two removed vapor compartments of the first evaporator and the second evaporator result in a combined vapor flow at the end of the first mechanically operated compressor unit, which can then be divided into a first partial flow for the first evaporator and a second partial flow for the second evaporator. It can be provided that vapor formed in the first evaporator is compressed downstream with several compressors of the first mechanically operated compressor unit. Several compressors can also be provided in the second compressor unit. The number of compressors is designed such that, on the one hand, the desired temperature rise for re-feeding the vapor into the evaporator is achieved, but on the other hand, energy consumption is minimized.

[0020] Within the scope of the invention, the steam in the second compressor unit can be compressed to a greater difference between the outlet temperature and the inlet temperature than in the first compressor unit. Typically, it is necessary to subject the steam exiting the second evaporator to a greater temperature increase, since not only does the second evaporator require a higher energy input, but the steam is also removed at a lower temperature.

[0021] In the context of the invention, the first evaporator is generally operated at a higher pressure than the second evaporator.

[0022] Further features, advantages, and effects of the invention will become apparent from the exemplary embodiment presented below. In the drawing, Fig. 1 shows a system concept according to the invention.

[0023] Fig. 1 shows a system 1 designed for concentrating NMMO. 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 fed to the first evaporator 2 for preconcentration with an NMMO content of, for example, 20 wt.%. In the first evaporator 2, the supplied NMMO solution is brought to an elevated temperature by the supply of energy. The resulting vapor can be removed from the top of the first evaporator 2.The pre-concentrated NMMO solution is removed at the bottom and fed to the second evaporator 3, in which the supplied solution is again brought to an elevated temperature, so that vapor can again be withdrawn at the top side of the second evaporator 3, resulting in another increase in concentration and finally an NMMO solution with 80 wt.% NMMO can be removed at the bottom side of the second evaporator 3, which serves as the final evaporator.

[0024] The steam exiting the first evaporator 2 is reused in the further process. A first mechanically operating compressor unit 4 is provided for this purpose. The first compressor unit 4 comprises several individual compressors 41. In the exemplary embodiment, the first compressor unit 4 comprises two compressors 41. The two series-connected compressors 41 of the first compressor unit 4 are connected to the first evaporator 2 via a first discharge line 42. Similarly, the second evaporator 3 is designed with a discharge line 52, which connects a head-side outlet of the second evaporator 3 to a second mechanically operating 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.

[0025] As can be seen in Fig. 1, the compressors 51 of the second compressor unit 5 are equipped downstream with a line that opens into the discharge line 42 of the first evaporator 2. Vapor compressed by the second compressor unit 5 is therefore combined with the vapor discharged from the first evaporator 2 before the combined vapor is further compressed in the first compressor unit 4. After this final compression, the compressed vapor is divided into two partial streams, namely a first partial stream for a first supply line 6 to the first evaporator 2 and a second supply line 7 to the second evaporator 3.

[0026] The circuit provided makes it possible for vapor from the second evaporator 3, which exits at a relatively low temperature of approximately 30°C less than from the first evaporator 2, to be initially brought to a temperature approximately 30°C to 40°C higher using the provided compressors 51 of the second compressor unit 5. This alone would not be sufficient in terms of temperature for the operation of the second evaporator 3. However, the additional compression with the first compressor unit 4 can achieve a further temperature increase of several °C, so that one temperature of the compressed vapor is sufficient to adequately heat the second evaporator 3. This means that a minimum number of mechanically operating compressors is sufficient, resulting in an energetically efficient process. As can be seen in Fig. 1, the NMMO solution can be concentrated to approximately 40 wt.% NMMO in the first evaporator 3 and finally to 80 wt.% NMMO in the second.-% NMMO can be concentrated.

Claims

Claims 1. A plant (1) for concentrating substance-containing liquids by multi-stage evaporation, in particular of solutions such as N-methylmorpholine-N-oxide (NMMO), wherein the plant (1) is designed 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) so that liquid concentrated in the first evaporator (2) can be fed into the second evaporator (3) in order to further concentrate the concentrated liquid in the second evaporator (3), wherein a first mechanically operating compressor unit (4) is provided, with which vapor formed in the first evaporator (2) can be compressed downstream, and wherein the plant (1) comprises a first supply line (6) for supplying vapor compressed by the first mechanically operating compressor unit (4) to the first evaporator (2), characterized in thatthat a second mechanically operating compressor unit (5) is provided, with which vapor formed in the second evaporator (3) can be compressed downstream, wherein vapor compressed by the second compressor unit (5) can be fed to the first compressor unit (4).

2. Plant (1) according to claim 1, characterized in that the first compressor unit (4) comprises several compressors (41).

3. Plant (1) according to claim 1 or 2, characterized in that the second compressor unit (5) comprises several compressors (51) 4. Plant (1) according to one of claims 1 to 3, characterized in that the plant (1) comprises a second supply line (6) for supplying steam compressed by the first mechanically operating compressor unit (4) into the second evaporator (3).

5. Plant (1) according to one of claims 1 to 4, characterized in that no thermal compressor is provided.

6. Plant (1) according to one of claims 1 to 5, characterized in that the second evaporator (3) is designed as a final evaporator from which the concentrated liquid is withdrawn from the plant (1).

7. A method for concentrating substance-containing liquids by multi-stage evaporation, for example of solutions such as N-methylmorpholine-N-oxide (NMMO), in particular with a device (1) according to one of claims 1 to 6, wherein the liquid is concentrated in a first evaporator (2) and the thus concentrated liquid is fed to a second evaporator (3) in which the concentrated liquid is further concentrated, after which a concentrate is withdrawn, wherein vapor formed in the first evaporator (2) is compressed downstream with a first mechanically operating compressor unit (4) and at least partially fed to the first evaporator (2) via a first supply line (6), characterized in that vapor formed in the second evaporator (3) is compressed downstream with a second mechanically operating compressor unit (5) and then fed to the first compressor unit (4).

8. Method according to claim 7, characterized in that the vapor compressed in the second evaporator (3) is compressed together with the vapor formed in the first evaporator (2) in the first mechanically operating compressor unit (4).

9. Method according to claim 7 or 8, characterized in that in the first mechanically operating compressor unit (4) compressed steam is divided into partial flows, wherein one partial flow is fed to the first evaporator (2) and another partial flow is fed to the second evaporator (3).

10. Method according to one of claims 7 to 9, characterized in that vapor formed in the second evaporator (3) is compressed downstream with a plurality of compressors (51) of the second mechanically operating compressor unit (5).

11. The method according to any one of claims 7 to 10, characterized in that vapor formed in the first evaporator (2) is compressed downstream with several compressors (41) of the first mechanically operating 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. Method according to one of claims 7 to 12, characterized in that steam is compressed in the second compressor unit (5) to a higher difference between outlet temperature and inlet temperature than in the first compressor unit (4).

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