Vacuum evaporation-concentration process and installation

Abstract

There are described a mechanical vapor recompression vacuum evaporation-concentration process and installation in which evaporation takes place at temperatures lower than 65°C and compression of the generated vapor is carried out by means of a high-performance turbofan.

Description

[0001] VACUUM EVAPORATION-CONCENTRATION PROCESS AND INSTALLATION DESCRIPTION

[0002] FIELD OF THE INVENTION

[0003] The present invention falls within the technical field of vacuum evaporation-concentration processes and installations. In particular, the invention relates to a mechanical vapor recompression vacuum evaporation-concentration process and installation.

[0004] STATE OF THE ART

[0005] As is known, vacuum evaporation-concentration processes are thermal separation processes carried out under vacuum conditions, i.e., at a pressure below atmospheric pressure, to lower the boiling temperature of a liquid mixture to be treated, thus reducing the amount of thermal energy required to obtain separation therefrom of the relatively more volatile components through evaporation. These processes and the related installations are today used in the treatment of industrial or domestic waste water, to reduce the volume thereof, for example to implement zero liquid discharge (ZLD) systems, and / or to separate pollutants and / or recover water and any valuable raw materials or by-products, as well as in various production sectors, such as the food, chemical, pharmaceutical, cosmetic, or herbal sectors, to obtain concentrates and / or distillates from process liquid mixtures.

[0006] Among vacuum evaporation-concentration processes, in particular mechanical vapor recompression (MVR) processes are known. These processes exploit the vapor generated in the process itself and subsequently mechanically compressed, so as to increase the pressure, and hence the temperature, thereof up to a predetermined value, as a heating means to maintain the mixture to be treated in boiling condition and allow evaporation to continue. Heating and evaporation of the mixture are obtained at the expense of the latent condensation heat of the compressed vapor, which condenses.

[0007] By exploiting the enthalpy content of the vapor generated in the process itself and consequently reducing the demand for thermal energy from outside, this approach allows performing vacuum evaporation-concentration processes that are particularly efficient in terms of energy consumption, i.e., having relatively low specific consumption, typically of the order of 50-60 W / l.

[0008] Vacuum evaporation-concentration installations implementing MVR processes are characterized by vacuum evaporation units comprising a boiling chamber, inside which the liquid mixture to be treated is brought to boiling condition under a vacuum, typically at temperatures of between 80°C and 100°C, to achieve evaporation of the relatively more volatile fractions, heat exchange means for heating the mixture by heat exchange with the evaporated fraction discharged from the boiling chamber and subsequently compressed and, at the same time, to achieve condensation of the evaporated fraction, and a vapor compression line hydraulically connecting the boiling chamber to the heat exchange means and comprising compression means to compress the vapor discharged from the boiling chamber. The heat exchange means can be arranged inside the boiling chamber, or else outside the boiling chamber and be hydraulically connected thereto via a specific recirculation line of the liquid mixture. In the first case, the heat exchange means usually consist of vertical shell and tube heat exchangers, configured to create falling film or rising film operating conditions. The compression means usually consist of volumetric compressors, in the case of small-size installations, or turbo-compressors in the case of medium- or large-size installations.

[0009] The aforesaid structure of known MVR vacuum evaporation-concentration installations makes them more sensitive to phenomena that often go along with the evaporation-concentration of liquid mixtures, such as, in particular, entrainment of liquid droplets in the vapor flow and precipitation of compounds from the liquid that is being concentrated.

[0010] In fact, the entrainment of liquid droplets in the vapor flow causes phenomena of mechanical and, depending on the nature of the liquid entrained, at times also chemical corrosion in the compression means of the vacuum evaporation unit. These phenomena lead to a decrease in performance or even failure and, as a whole, to a reduction of the useful life of such components and higher expenses for their maintenance or replacement.

[0011] Moreover, the precipitation of compounds from the liquid being concentrated, in particular salts having retrograde solubility (i.e., a solubility that decreases as the temperature increases) in aqueous mixtures, such as some carbonates, sulphates and phosphates, causes fouling of the heat exchange means of the vacuum evaporation unit, which progressively worsen heat exchange and hence the efficiency of the processes of evaporation of the mixture and of condensation of the compressed vapor. This is particularly apparent and awkward in shell and tube heat exchangers, as this configuration, on the one hand, facilitates the formation of deposits and encrustations and, on the other hand, requires more burdensome cleaning operations in terms of complexity and time required.

[0012] These drawbacks represent a limitation to full exploitation of the potential of MVR vacuum evaporation-concentration processes and of the related installations.

[0013] SUMMARY OF THE INVENTION

[0014] In view of the above, it is an object of the present invention to provide an MVR vacuum evaporation-concentration process and installation that makes it possible to avoid, or at least substantially reduce, the drawbacks associated with entrainment and precipitation phenomena that go along with the evaporation-concentration of liquid mixtures.

[0015] Another object of the invention is to provide an MVR vacuum evaporation-concentration process and installation whose performance in terms of energy efficiency is comparable to, or possibly also higher than, those of known installations and processes.

[0016] A further object of the invention is to provide an MVR vacuum evaporation-concentration installation which also has a configuration that is relatively simple and easy to maintain and can be manufactured at competitive costs.

[0017] The Applicant has observed that the entrainment and precipitation phenomena that go along with the evaporation-concentration of liquid mixtures are linked to the process conditions, in particular to the boiling temperature, and has perceived the possibility of achieving all the aforesaid objects by acting on these conditions and by appropriately choosing the compression means so as to ensure optimal performance also in the process conditions identified as more suitable to tackle the aforesaid entrainment and precipitation phenomena.

[0018] In particular, in a first aspect thereof, the invention thus relates to a vacuum evaporationconcentration process for treating liquid mixtures, comprising the steps: feeding a mixture to be treated to a vacuum evaporation unit comprising a boiling chamber and heat exchange means; heating, by means of the heat exchange means, the mixture so as to cause a fraction thereof to evaporate in the boiling chamber; discharging the evaporated fraction from the boiling chamber; mechanically compressing the discharged evaporated fraction; feeding the compressed evaporated fraction to the heat exchange means; exchanging heat, by means of the heat exchange means, between the compressed evaporated fraction and the mixture so as to cause the compressed evaporated fraction to condense, thereby heating the mixture in the heating step; discharging a condensed evaporated fraction of the mixture from the heat exchange means; discharging a concentrated fraction of the mixture from the boiling chamber; wherein the evaporated fraction discharged from the boiling chamber and mechanically compressed has a temperature lower than 65 °C, and wherein the mechanical compression step is carried out by means of a mixed-flow turbofan.

[0019] Within the framework of the present description and of the subsequent claims, by the term “liquid mixture”, or briefly “mixture”, it is meant a generic aggregate of two or more substances comprising at least one liquid phase. In particular, this term includes both homogeneous aggregates, such as liquid solutions, and heterogeneous aggregates, such as emulsions or suspensions.

[0020] The process of the invention involves boiling, and hence producing and compressing vapor, at relatively low temperatures, in particular at least lower than 65°C. By limiting phenomena of rapid and violent evaporation, low boiling temperatures also reduce the entrainment of droplets of unevaporated liquid, foams and any solid particles in the vapor flow, and are also beneficial for contrasting the precipitation of salts having retrograde solubility.

[0021] Thanks to the use of a mixed-flow turbofan, i.e., a turbofan in which the impeller has a mixed radial / axial geometry, also in the aforesaid conditions it is possible to process high vapor flow rates and, at the same time, obtain high pressure increases, achieving an overall performance, in terms of throughput and energy efficiency, comparable to or, surprisingly, even higher than those typical of known MVR vacuum evaporation-concentration processes and installations.

[0022] At the same time, adopting lower boiling temperatures improves the energy balance of the process, due both to a lower consumption of primary energy to be supplied from the outside, and to lower dissipation losses.

[0023] Lower boiling temperatures also positively affect the costs of the installation, since it becomes possible to select components and materials which are less performing in terms of thermomechanical strength, and hence less expensive, and the need for thermal insulation is reduced, as well as the range of products that can be treated, since it becomes possible to process also thermolabile substances, as often required in applications in the food, chemicalpharmaceutical, and herbal sectors.

[0024] In preferred embodiments of the process, the evaporated fraction discharged from the boiling chamber and mechanically compressed has a temperature of between 40°C and 55°C.

[0025] The turbofan is operated at a speed preferably equal to or higher than 3000 rpm, more preferably of between 8000 rpm and 15000 rpm.

[0026] Preferably, the heat exchange step is carried out in said boiling chamber and said heat exchange means comprise a pillow-plate heat exchanger (PPHE) which is arranged inside said boiling chamber and in operation is at least partially submerged by, i.e. immersed in, said mixture.

[0027] Immersed pillow-plate heat exchangers are particularly effective for obtaining condensation of the compressed vapor and heating the mixture to be treated, also in conditions of low boiling temperatures. By arranging the pillow plates vertically and at a suitable distance from one another, it is also possible to generate convective motions in vertical direction in the boiling mixture, wherein said motions, beside improving heat exchange, promote an equalization of the conditions of the mixture inside the boiling chamber, avoiding in particular the development of areas of higher concentration, where the possibility of buildup or precipitation is increased. Moreover, the particular structure of these heat exchangers makes cleaning and maintenance thereof considerably easier compared to other types of heat exchangers, such as shell and tube heat exchangers, commonly used in MVR evaporation-condensation processes and installations of the prior art. In particular, cleaning of pillow-plate heat exchangers can advantageously be automated and carried out also in operation, by using suitably designed automated mechanical cleaning systems installed inside the boiling chamber of the vacuum evaporation unit.

[0028] Additionally, pillow-plate heat exchangers can be obtained through relatively simple and inexpensive manufacturing processes.

[0029] Thanks to the arrangement of the pillow-plate heat exchanger inside the boiling chamber it is possible to omit a special circuit with associated circulation pump that would be otherwise necessary for hydraulically connecting the boiling chamber to external heat exchange means. Moreover, this solution is advantageous for the purposes of recirculating the mixture present in the boiling chamber, as it allows a natural circulation to be established inside said chamber, thereby reducing the need for a recirculation outside the boiling chamber and allowing smaller recirculation pumps to be used for this purpose.

[0030] In this case, the process may comprise a step of extracting from the heat exchange means a part of the compressed evaporated fraction that is not yet or not fully condensed in the heat exchange means and returning it to the boiling chamber.

[0031] It can thus be avoided wasting condensation enthalpy of vapor that possibly is not condensed in the heat exchange means and would otherwise be discharged therefrom together with condensate. At the same time, the vapor returned to the boiling chamber can advantageously enhance the flow of the evaporated fraction generated therein toward a corresponding discharge opening for subsequent compression.

[0032] According to a preferred embodiment, the process further comprises the steps: preheating the mixture before feeding it to the vacuum evaporation unit, cooling the condensed evaporated fraction discharged from the heat exchange means, and these preheating and cooling steps are carried out by means of heat exchange with a working fluid circulating in a refrigeration circuit implementing a heat pump cycle, wherein the working fluid absorbs heat from the condensed fraction in the cooling step and releases heat to the mixture in the preheating step.

[0033] In the process of the invention, due to the lower operating temperatures, it is possible to recover heat from the condensate obtained and use it for preheating the mixture to be treated in a particularly effective way by means of a heat pump system, which ensures greater operating flexibility and high energy efficiency compared to solutions based on direct heat exchange by means of economizer typically used in known MVR vacuum evaporation-concentration processes. The same heat pump system can also be effectively used to heat the mixture at startup.

[0034] Moreover, preferably, said refrigeration circuit is in heat exchange relationship with a vacuumgenerating unit hydraulically connected to the vacuum evaporation unit and the working fluid circulating in the refrigeration circuit further absorbs heat from the vacuum-generating unit, preferably before heat exchange with the condensed evaporated fraction in said cooling step. In this way, it is possible to cool the condensate and the vacuum-generating unit and recover the corresponding heat for preheating the mixture to be treated with a single refrigeration circuit, thereby optimizing the installation configuration.

[0035] In a second aspect thereof, the invention relates to a vacuum evaporation-concentration installation for treating liquid mixtures provided with a vacuum evaporation unit comprising: a boiling chamber intended to receive a mixture to be treated in boiling condition under a vacuum; heat exchange means intended to heat the mixture for reaching said boiling condition, and a vapor compression line hydraulically connecting a vapor discharge opening of the boiling chamber to the heat exchange means and comprising compression means for compressing an evaporated fraction of said mixture discharged from the boiling chamber, wherein said heat exchange means comprise a mixed-flow turbofan.

[0036] This installation is adapted to carry out the vacuum evaporation-condensation process described above and has similar advantages.

[0037] Preferably, the compression means are connected to an inlet end of the vapor compression line at the vapor discharge opening of the boiling chamber.

[0038] Preferably, the heat exchange means comprise a pillow-plate heat exchanger arranged inside the boiling chamber in a region at least partially floodable with said mixture.

[0039] Preferably, the pillow-plate heat exchanger comprises vapor discharge means hydraulically connected to at least some of its pillow plates and having a discharge opening which opens into the boiling chamber.

[0040] Preferably, the installation further comprises a refrigeration circuit configured to subject a working fluid to a heat pump cycle. The refrigeration circuit comprises condensation means, evaporation means, compression means, and expansion means, wherein:

[0041] - the condensation means comprise a first heat exchange device arranged in a feed line of the vacuum evaporation unit, in which the working fluid is set in heat exchange relationship with said mixture to be treated before feeding it to the boiling chamber, and

[0042] - the evaporation means comprise a second heat exchange device arranged in a condensate discharge line of the vacuum evaporation unit, in which the working fluid is set in heat exchange relationship with the condensed evaporated fraction discharged from the heat exchange means of the vacuum evaporation unit.

[0043] Preferably, the installation comprises a vacuum-generating unit hydraulically connected to the vacuum evaporation unit and provided with a cooling circuit for cooling a respective working liquid, and said evaporation means comprise a third heat exchange device arranged in the cooling circuit of the vacuum-generating unit, in which the working fluid is set in heat exchange relationship with said working liquid.

[0044] LIST OF FIGURES

[0045] Further features and advantages of the invention shall become more apparent from the following detailed description of preferred embodiments thereof, provided hereinafter for indicating and non-limiting purposes with reference to the accompanying drawings, wherein:

[0046] Fig. 1 is a conceptual diagram of a vacuum evaporation-concentration installation according to the present invention, and

[0047] Fig. 2 is a schematic perspective sectional view of a vacuum evaporation unit of the vacuum evaporation-concentration installation of Fig. 1.

[0048] DETAILED DESCRIPTION OF THE INVENTION

[0049] Referring in particular to Fig. 1, a vacuum evaporation-concentration installation according to the present invention is generally indicated by reference numeral 1.

[0050] The installation 1 is adapted to carry out a mechanical vapor recompression (MVR) vacuum evaporation-concentration process according to the invention, described more in detail below, to thermally separate from a liquid mixture, hereinafter also generically indicated by the term “product”, an evaporated and subsequently condensed fraction, or distilled fraction, containing the relatively more volatile component / components, and a concentrated fraction, containing the relatively less volatile component / components. The mixture to be treated can generally be industrial or domestic waste water, such as, for example, a process mixture used or produced in an industrial process, waste water from a washing process of an installation, a reject from separation processes carried out by means of reverse osmosis or exchange resins, a digestate from biogas or compost production processes, or from the treatment of livestock waste, a landfill leachate, as well as a process mixture to be concentrated and / or distilled in food, chemical-pharmaceutical, herbal, or cosmetic production processes.

[0051] The installation 1 essentially comprises a vacuum evaporation unit 100 of mechanical vapor recompression type, a vacuum-generating unit 200 for generating and maintaining a predetermined degree of vacuum in the vacuum evaporation unit 100, and a refrigeration circuit 300 in which a working fluid can be subjected to a thermodynamic heat pump cycle for cooling the vacuum -generating unit 200 and the condensed vapor discharged from the vacuum evaporation unit 100, and at the same time preheating the product fed to the vacuum evaporation unit 100.

[0052] The vacuum evaporation unit 100, shown in more detail in Fig. 2, comprises a boiling chamber 110, inside which the product to be treated is brought to a boiling condition under a vacuum and partially evaporated, heat exchange means 120 intended to heat the product to be treated to cause the aforesaid boiling by means of heat exchange with the generated and subsequently compressed vapor, and a vapor compression line 130.

[0053] The boiling chamber 110 preferably comprises a lower cylindrical body 111 having a horizontal longitudinal axis and an upper cylindrical body 112 having a vertical longitudinal axis, mutually superimposed and directly connected to each other.

[0054] The lower cylindrical body 111 is closed at its ends by hemispheric side walls I l la, 111b and delimits therewith an inner space intended to be partially flooded by the product to be treated in boiling condition, on the bottom of which the concentrated fraction is collected. The horizontal arrangement of the lower cylindrical body 111 advantageously allows a wide boiling surface to be maintained, which helps to reduce boiling turbulence and hence the formation of foams and the entrainment of liquid droplets in the vapor. One of the side walls I l la, 11 lb, in the present case side wall I l la, defines an openable hatch that allows accessing said inner space for inspection, cleaning and / or maintenance.

[0055] The upper cylindrical body 112 has a first end connected to the lower cylindrical body 111 and a second end closed by a hemispheric upper wall 112a and defines a collection dome for the vapor resulting from boiling the product to be treated in the lower cylindrical body 111.

[0056] The boiling chamber 110 comprises a product feed opening IP1, for feeding the product to be treated, a recirculation opening IP2, for recirculating the product undergoing concentration and temporarily accumulated in the boiling chamber 110, a compressed vapor feed opening IP3, a vapor discharge opening OP1, a condensate discharge opening OP2, and a concentrate discharge opening OP3. The product feed opening IP1 (not visible in Fig. 2), the recirculation opening IP2, and the concentrate discharge opening OP3 are preferably formed in the lower cylindrical body 111, whereas the compressed vapor feed opening IP3 and the condensate discharge opening OP2 are preferably formed in the side wall 111b. The vapor discharge opening OP1 is preferably formed in the upper wall 112a of the upper cylindrical body 112. The heat exchange means 120 of the vacuum evaporation unit 100 are preferably configured as immersed pillow-plate heat exchanger and are advantageously located inside the boiling chamber 110. More in detail, the heat exchange means 120 are arranged inside the lower cylindrical body 111 of the boiling chamber 110, in a position allowing them to be at least partially submerged by the product received inside the lower cylindrical body 111.

[0057] In this case, the heat exchange means 120 thus comprise an array of pillow plates 121, preferably vertically arranged and preferably oriented parallel to the direction of longitudinal development of the lower cylindrical body 111. The pillow plates 121 can have, in a known way, a single- or double-embossed configuration. The pillow plates 121 are hydraulically connected to one another in parallel by means of a manifold 122, intended to feed the compressed vapor to, and to discharge the condensed vapor from, each pillow plate 121. For this purpose, the manifold 122 is hydraulically connected to the compressed vapor feed opening IP3 and to the condensate discharge opening OP2 of the boiling chamber 110 by means of respective conduits 123 and 124.

[0058] The pillow-plate heat exchanger is preferably provided with vapor discharge means 125 for extracting therefrom not yet or not fully condensed vapor and returning it to the boiling chamber 110. The vapor discharge means 125, which are only schematically shown in the figures, may be configured as a hood hydraulically connected to at least some of the pillow plates 121, for example at an upper side of corresponding array, and having a discharge opening 126 that opens into the boiling chamber 110. However, the exact configuration and position of the vapor discharge means 125 may be selected based on the specific functional needs and / or structural constraints.

[0059] When the heat exchange means 120 are configured as pillow-plate heat exchanger, as described above, an automated mechanical cleaning system (not shown in the figures) can advantageously be provided inside the boiling chamber 110 to carry out automated cleaning of the pillow-plate heat exchanger. Such a cleaning system may for example comprise brushing and / or scraping elements movable along and in contact with the outer surfaces of the pillow plates 121, fitted onto support arms or rods actuated so as to carry out reciprocating movements parallel to said surfaces.

[0060] Installation configurations in which the heat exchange means 120 are arranged outside the boiling chamber 110 and hydraulically connected thereto by means of a recirculation line, and / or in which the heat exchange means 120 have a different configuration, for example conventional a shell and tube configuration, are nevertheless not excluded.

[0061] The vapor compression line 130 extends between the vapor discharge opening OP1 and the compressed vapor feed opening IP3 of the boiling chamber 110 and comprises compression means 131 for compressing the vapor discharged from the boiling chamber 110.

[0062] The compression means are configured as a high-speed, mixed-flow turbofan. In particular, the turbofan is capable of operating at a speed of at least 3000 rpm, and preferably of between 8000 rpm and 15000 rpm.

[0063] The turbofan is driven by an electric motor, preferably directly, i.e., the impeller is directly fitted to the drive shaft of the electric motor. The electric motor is preferably provided with an inverter for controlling the number of revolutions.

[0064] Preferably, the turbofan is connected to an inlet end of the vapor compression line 130 at the vapor discharge opening OP1. In particular, the turbofan can advantageously be arranged directly above the boiling chamber 110 and be substantially supported thereby.

[0065] The vacuum evaporation unit 100 is fed with the product to be treated by means of a feed line 140 connected to the product feed opening IP1 of the boiling chamber 110. The feed line 140 preferably comprises a main branch 140a and a by-pass branch 140b.

[0066] The main branch 140a of the feed line 140 is provided with a feed valve VI and preheating means for preheating the product to be treated before it enters the boiling chamber 110.

[0067] The preheating means preferably comprise a preheating heat exchanger 141, preferably of the shell and tube type, in which the product to be treated is set in heat exchange relationship with the working fluid circulating in the refrigeration circuit 300. As described in more detail below, the latter is configured so as to achieve condensation of the working fluid at the preheating heat exchanger 141, while correspondingly heating the product to be treated. Preferably, the preheating heat exchanger 141 is connected to the main branch 140a of the feed line 140 and to the refrigeration circuit 300 so that the product to be treated flows on the tube side and the working fluid flows on the shell side.

[0068] In alternative embodiments, the preheating of the product to be treated, rather than by means of heat recovered from other steps of the evaporation-condensation process carried out in the installation 1, could be achieved by means of hot water or vapor generated for this purpose or available from other processes and fed to the preheating heat exchanger 141. Moreover, the preheating means could be located inside the boiling chamber 110 or integrated therewith. Advantageously, the preheating means can also comprise one or more auxiliary heating elements (not shown in the figures), for example in the form of electrical resistors, possibly integrated in the preheating heat exchanger 141. The auxiliary heating elements can be used to complement preheating, for example at the installation start-up, to reach steady state operation more rapidly, and / or in particular operating conditions in which the heat recovered in the evaporation-condensation process is not sufficient to maintain a required preheating level.

[0069] The by-pass branch 140b of the feed line 140, which can be activated by means of a corresponding feed valve V2, allows the preheating heat exchanger 141 to be by-passed.

[0070] The concentrated fraction that gradually accumulates in the boiling chamber 110 is discharged by means of a concentrate discharge line 150 connected to the concentrate discharge opening OP3 and comprising a concentrate pump Pl and a discharge valve V3.

[0071] A recirculation line 160 allows recirculation of the product that accumulates in the boiling chamber 110 both in steady state operation, and at start-up of the installation 1. For this purpose, the recirculation line 160 is connected to the concentrate discharge line 150 at a point downstream of the concentrate pump Pl and upstream of the discharge valve V3, and branches into a first branch 160a, connected to the recirculation opening IP2 of the boiling chamber 110 and provided with a respective shut-off valve V4, and a second branch 160b, connected to the main branch 140a of the product feed line 140, downstream of the feed valve VI and upstream of the preheating heat exchanger 141, and provided with a respective shut-off valve V5. Recirculation is carried out by means of the concentrate pump Pl, in steady state operating condition by opening the valve V4 and keeping the valves V3 and V5 closed, and at start-up by opening the valve V5 and keeping the valves VI, V3, and V4 closed.

[0072] The evaporated and condensed fraction, or distilled fraction, is discharged from the vacuum evaporation unit 100 by means of a condensate discharge line 170 connected to the condensate discharge opening OP2 of the boiling chamber 110 and comprising a condensate cooling tank 171, a condensate pump P2 arranged downstream of the condensate cooling tank 171, and a discharge valve V6 arranged downstream of the condensate pump P2.

[0073] Heat exchange means 172, for example configured as a plate heat exchanger, or as a tube bundle or coil, are installed in the condensate cooling tank 171 for setting the condensate accumulated therein in heat exchange relationship with the working fluid circulating in the refrigeration circuit 300. As described more in detail below, the latter is configured so as to obtain at least partial evaporation of the working fluid in the heat exchange means 172, causing a corresponding cooling of the condensate.

[0074] The vacuum-generating unit 200 comprises a vacuum pump 201. In the preferred embodiment of the installation 1 illustrated herein, the vacuum pump 201 is preferably a liquid ring vacuum pump and the vacuum-generating unit 200 also comprises a recirculation circuit 202 of a respective working liquid, typically water. The recirculation circuit 202 is provided with a working liquid cooling tank 203, in which heat exchange means 204 are installed, for example configured as a plate heat exchanger, or as a tube bundle or coil. By means of the heat exchange means 204 the working liquid in the working liquid cooling tank 203 is set in heat exchange relationship with the working fluid circulating in the refrigeration circuit 300, so as to be cooled thereby during an evaporation step thereof.

[0075] In all cases, the vacuum-generating unit 200 is hydraulically connected to the vacuum evaporation unit 100, to create and maintain therein a predetermined degree of vacuum. More in detail, in the preferred embodiment of the installation 1 illustrated herein, the suction of the vacuum pump 201 is connected to the condensate discharge line 170 upstream of the pump P2, in particular at the condensate cooling tank 171.

[0076] As already mentioned above, the refrigeration circuit 300 is configured to implement a thermodynamic heat pump cycle in which the working fluid circulating therein evaporates after having undergone an expansion, absorbing heat from the condensate discharged from the vacuum evaporation unit 100 and from the working liquid of the vacuum -generating unit 200, and condensates after having being subjected to compression, releasing heat to the product to be treated so as to preheat it before being fed to the vacuum evaporation unit 100.

[0077] For this purpose, the refrigeration circuit 300 comprises in a known way condensation means, evaporation means, compression means 301, and expansion means 302 hydraulically connected to one another, wherein the condensation means comprise the preheating heat exchanger 141 provided in the main branch 140a of the feed line 140, and the evaporation means comprise the heat exchange means 172 provided in the condensate cooling tank 171, as well as the heat exchange means 204 provided in the working liquid cooling tank 203 of the vacuum-generating unit 200. Accordingly, the compression means 301 are arranged in the refrigeration circuit 300 so that their suction side is hydraulically connected to the heat exchange means 172 and the heat exchange means 204 and their discharge side is hydraulically connected to the preheating heat exchanger 141, and the expansion means 302 are arranged in the refrigeration circuit 300 so that their inlet side is hydraulically connected to the preheating heat exchanger 141 and their outlet side is hydraulically connected to the heat exchange means 172 and the heat exchange means 204. The heat exchange means 172 and the heat exchange means 204 are hydraulically connected in series to each other, preferably so that the heat exchange means 204 are located upstream of the heat exchange means 172 considering the normal flow direction (indicated by arrows in Fig. 1) of the working fluid in the refrigeration circuit 300.

[0078] The working fluid circulating in the refrigeration circuit 300 is a coolant that can be selected from those conventionally known for use in refrigeration circuits, preferably a HFC or HFO with low environmental impact and low GWP.

[0079] Still referring to Fig. 1, a preferred embodiment of a vacuum evaporation-concentration process for treating liquid mixtures that can be carried out in the installation 1 illustrated above shall now be described.

[0080] In steady state operating condition, the product to be treated, typically at ambient temperature and pressure, is fed to the vacuum evaporation unit 100 through the main branch 140a of the feed line 140. Feeding takes place either discontinuously or slowly continuously, by controlling the valve VI, and is preferably carried out so as to stably maintain in the lower region of the boiling chamber 110, defined by the lower cylindrical body 111 and the respective side walls I l la, 111b, an amount of product corresponding to about 50% of the volume of said region. This makes it possible to ensure proper immersion of the heat exchange means 120, in particular of the array of pillow plates 121, in the product for causing boiling thereof, and, at the same time, to provide a sufficiently wide boiling surface. Feeding is preferably carried out automatically based on the level of liquid in the boiling chamber 110, detectable by means of one or more level sensors (not shown).

[0081] In the feeding step, the product is preheated in the preheating heat exchanger 141, by means of the heat recovered from cooling the evaporated and condensed fraction discharged from the vacuum evaporation unit 100 and the working liquid of the vacuum-generating unit 200, and transferred to the fed product by means of the heat pump system based on the refrigeration circuit 300 described above.

[0082] By means of the preheating, the product is brought to a temperature Ti preferably of between 30°C and 45°C.

[0083] Of course, if the product to be treated already is at a temperature sufficient for reaching vacuum boiling in the boiling chamber 110 with the sole contribution of the condensation heat of the recompressed vapor, the aforesaid preheating can be avoided and the product can be fed directly to the boiling chamber 110, through the by-pass branch 140b of the feed line 140. In this case, the heat generated by cooling the evaporated and condensed fraction discharged from the vacuum evaporation unit 100 and the working liquid of the vacuum -generating unit 200 can be released to the outer environment through one or more working fluid - air heat exchangers (not shown in the figures) provided in the refrigeration circuit 300 for this purpose.

[0084] Inside the boiling chamber 110, the product fed is further heated by means of the heat exchange means 120, in particular configured as immersed pillow-plate heat exchanger, as previously described, until reaching the boiling temperature corresponding to the degree of vacuum maintained in the vacuum evaporation unit 100 by means of the vacuum -generating unit 200.

[0085] In the typical case in which the fraction to be separated through evaporation essentially consists of water, a pressure peof between around 0.900 mbar and around 0.950 mbar is maintained in the vacuum evaporation unit 100, and in particular inside the boiling chamber 110, and the boiling temperature Teis preferably of between 40°C and 55°C, and in any case lower than 65°C.

[0086] Further to vacuum boiling, vapor is generated in the boiling chamber 110 and is discharged, preferably continuously, from the boiling chamber 110 through the vapor discharge opening OP1.

[0087] According to the typical operating principle of mechanical vapor recompression vacuum evaporation-concentration installations, the discharged vapor is mechanically compressed to a predetermined pressure value by means of the compression means 131, thereby obtaining a corresponding predetermined temperature increase relative to the boiling temperature in the boiling chamber 110. The compressed vapor is then fed to the heat exchange means 120 of the vacuum evaporation unit 100 through the vapor compression line 130, to complete heating of the fed product up to the boiling temperature, as described above, and, at the same time, obtain condensation of the compressed vapor.

[0088] More in detail, by means of the compression means 131, configured as a mixed-flow turbofan having the features described above, the discharged vapor is compressed with high efficiency at a pressure p2 preferably of between 30 mbar and 50 mbar, thereby reaching a temperature T2 preferably of between 45°C and 65°C. In general, compression should ensure a temperature increase of between 4°C and 10°C relative to the boiling temperature Teat which the vapor is discharged from the boiling chamber 110. To process large vapor flow rates, the turbofan is operated at high speed, in particular greater than 3000 rpm, and preferably of between 8000 rpm and 15000 rpm.

[0089] The condensed vapor leaves the heat exchange means 120 of the vacuum evaporation unit 100 preferably in saturated liquid conditions, at a temperature T3 preferably of between 40°C and 55°C.

[0090] As the compressed vapor flows in the heat exchange means 120 a part of it that is not yet condensed or possibly does not fully condense in the heat exchange means 120 can be extracted therefrom through the vapor discharge means 125 and returned to the boiling chamber 110, preferably so as to be directed toward the upper cylindrical body 112, thereby enhancing the flow of the evaporated fraction of boiling mixture toward the vapor discharge opening OP1. The condensed vapor thus obtained is cooled, preferably to temperatures close or equal to ambient temperature, in the condensate cooling tank 171 arranged in the condensate discharge line 170, by means of heat exchange with the working fluid circulating in the refrigeration circuit 300, and the recovered heat is used for preheating the fed product, as described above. Finally, the cooled condensed vapor is discharged from the condensate cooling tank 171 by opening the discharge valve V6 and activating the condensate pump P2. Discharge of the condensed vapor can take place automatically, for example based on the level of condensate in the condensate cooling tank 171, detectable by means of a level sensor (not shown).

[0091] The fraction of product that does not evaporate concentrates and settles at the bottom of the lower cylindrical body 111 of the boiling chamber 110, wherefrom it is discharged through the concentrate discharge line 150 by opening the discharge valve V3 (valves V4 and V5 closed) and activating the concentrate pump Pl. Discharge of the concentrate can take place automatically, preferably discontinuously, for example based on the density of the liquid phase in the lower cylindrical body 111 of the boiling chamber 110, detectable by means of a density meter (not shown).

[0092] Preferably, during boiling, the fraction of the product being concentrated is recirculated outside the boiling chamber 110, through the first branch 160a of the recirculation line 160, by opening the valve V4 (valves V3 and V5 closed) and activating the concentrate pump Pl. Recirculation can be carried out automatically, preferably discontinuously, at settable time intervals.

[0093] Start-up of the installation 1 for reaching steady state operating conditions can be performed through the following steps:

[0094] - activating the vacuum-generating unit 200 to create and maintain a predetermined degree of vacuum in the boiling chamber 110;

[0095] - activating the refrigeration circuit 300 and possibly the auxiliary heating elements of the preheating meansl41 in the feed line 140;

[0096] - feeding a first batch of product to the boiling chamber 110 through the main branch 140a or the by-pass branch 140b of the feed line 140;

[0097] - recirculating the fed product outside the boiling chamber 110 through the second branch 160b of the recirculation line 160 and the main branch 140a of the feed line 140 for gradually heating the product by means of the heat transferred through the refrigeration circuit 300, optionally complemented by heat generated by the auxiliary heating elements, so as to trigger vacuum boiling and vapor generation in the boiling chamber 110.

[0098] The invention thus provides an MVR vacuum evaporation-concentration process and installation that makes it possible to operate with low boiling / evaporation temperatures, considerably reducing adverse effects linked to entrainments in the vapor flow and to the precipitation of substances and fouling of critical components of the installation, and to ensure at the same time excellent performance in terms of energy consumption and throughput. From the energy point of view, in particular, tests performed by the Applicant have shown that the process and installation described above allow achieving specific energy consumption of between 20 and 40 W / l, up to 50% lower than energy consumption of known MVR vacuum evaporation-condensation installations of the same size.

[0099] Those skilled in the art can make changes to the installation and to the process described above in order to meet specific and contingent application needs, said changes in any case falling within the scope of protection as defined by the appended claims.

Claims

CLAIMS1. Vacuum evaporation-concentration process for treating liquid mixtures, comprising the steps:- feeding a mixture to be treated to a vacuum evaporation unit (100) comprising a boiling chamber (110) and heat exchange means (120);- heating, by means of said heat exchange means (120), the mixture so as to cause a fraction thereof to evaporate in said boiling chamber (110);- discharging the evaporated fraction from said boiling chamber (110);- mechanically compressing the discharged evaporated fraction;- feeding the compressed evaporated fraction to said heat exchange means (120);- exchanging heat, by means of said heat exchange means (120), between the compressed evaporated fraction and the mixture so as to cause the compressed evaporated fraction to condense, thereby heating the mixture in said heating step;- discharging a condensed evaporated fraction of the mixture from said heat exchange means (120), and- discharging a concentrated fraction of the mixture from said boiling chamber (110), wherein the evaporated fraction discharged from said boiling chamber (110) and mechanically compressed has a temperature lower than 65°C, and wherein said mechanical compression step is carried out by means of a mixed-flow turbofan (131).

2. Process according to claim 1, wherein the evaporated fraction discharged from said boiling chamber (110) and mechanically compressed has a temperature of between 40°C and 55°C.

3. Process according to any one of the previous claims, wherein said turbofan (131) is operated at a speed higher than 3000 rpm.

4. Process according to any one of the previous claims, wherein said heat exchange step is carried out in said boiling chamber (110) and said heat exchange means (120) comprise a pillow-plate heat exchanger (121, 122) arranged inside said boiling chamber (110) and at least partially submerged by said mixture.

5. Process according to any one of the previous claims, further comprising the steps:- preheating the mixture before feeding it to the vacuum evaporation unit (100),- cooling the condensed evaporated fraction discharged from the heat exchange means (120), wherein said preheating step and said cooling step are carried out through heat exchangewith a working fluid circulating in a refrigeration circuit (300) implementing a heat pump cycle, wherein the working fluid absorbs heat from the condensed evaporated fraction in said cooling step and releases heat to said mixture in said preheating step.

6. Process according to claim 5, wherein said refrigeration circuit (300) is in heat exchange relationship with a vacuum-generating unit (200) hydraulically connected to the vacuum evaporation unit (100) and the working fluid circulating in said refrigeration circuit (300) further absorbs heat from said vacuum-generating unit (200), preferably before heat exchange with the condensed evaporated fraction in said cooling step.

7. Vacuum evaporation-concentration installation (1) for treating liquid mixtures provided with a vacuum evaporation unit (100) comprising:- a boiling chamber (110) intended to receive a mixture to be treated in boiling condition under a vacuum;- heat exchange means (120) intended to heat said mixture for reaching said boiling condition, and- a vapor compression line (130) hydraulically connecting a vapor discharge opening (OP1) of said boiling chamber (110) to said heat exchange means (120) and comprising compression means (131) for compressing an evaporated fraction of said mixture discharged form said boiling chamber (110), wherein said compression means comprise a mixed-flow turbofan (131).

8. Installation (1) according to claim 7, wherein said compression means (131) are connected to an inlet end of said vapor compression line (130) at said vapor discharge opening (OP1).

9. Installation (1) according to claim 7 or 8, wherein said heat exchange means (120) comprise a pillow-plate heat exchanger (121, 122) arranged inside said boiling chamber (110) in a region at least partially floodable with said mixture.

10. Installation (1) according to any one of claims 7 to 9, further comprising a refrigeration circuit (300) configured to subject a working fluid to a heat pump cycle, wherein said refrigeration circuit (300) comprises condensation means, evaporation means, compression means (301), and expansion means (302), and wherein:- said condensation means comprise a first heat exchange device (141) arranged in a feed line (140) of the vacuum evaporation unit (100), in which the working fluid is set in heat exchange relationship with said mixture before feeding it to said boiling chamber (100), and- said evaporation means comprise a second heat exchange device (172) arranged in acondensate discharge line (170) of the vacuum evaporation unit (100), in which the working fluid is set in heat exchange relationship with a condensed evaporated fraction discharged from said heat exchange means (120) of the vacuum evaporation unit (100).

11. Installation (1) according to claim 10, further comprising a vacuum-generating unit (200) hydraulically connected to the vacuum evaporation unit (100) and provided with a cooling circuit (202, 203) for cooling a respective working liquid, wherein said evaporation means comprise a third heat exchange device (204) arranged in said cooling circuit (202, 203), in which the working fluid is set in heat exchange relationship with said working liquid.

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