DESCRIPTION

Field of the invention [0001] The present invention relates to a process for removing water from a mixture containing a compound of interest, for example a liquid mixture such as a solution of an acid or of a salt, or a more or less concentrated dispersion of a solid. The process is also suitable for removing water from a wet solid such as a salt, possibly until crystallization occurs, or from a sludge, for instance, a waste sludge from an industrial process.

Description of the prior art

[0002] Methods are known for removing water from a mixture by evapora tion. These methods have the drawback of requiring a large amount of heat. Moreover, in these methods the heat must be available at a temperature rea sonably higher than the boiling point the water, in order to establish an accepta ble heat exchange rate through a heat exchange surface that is necessarily small. Such improvements as the use of serially arranged evaporators operating at respective vacuum degrees different from one another, or the use of thermo compression, despite most recent developments, mitigate only in part these drawbacks.

[0003] As an alternative to evaporation, WO/2018/142349 describes a pro cess for removing water from a liquid by bubbling a gas containing ammonia therethrough. Flowever, this process cannot be used to treat acid mixtures, in particular it does not allow concentrating acid dilute solutions. Actually, this pro- cess is based on the equilibrium reaction:

NH ₃ (g) + H2O (I) ¾ NH4OH (I).

Due to the supply of NH3, this reaction is appreciably shifted towards the right- hand side, i.e. towards the removal of the water and the production of ammoni um hydroxide, but this occurs only if the pH is higher than 7, the higher the pH, the better the process works. In the opposite case, i.e. under acid conditions, the removal of water from the mixture does not practically take place. [0004] In particular, the mentioned process does not make it possible to concentrate dilute acid solutions, in which case the treatment with ammonia causes ammonium salts, such as sulphates, carbonates, chlorides, and the like, to be formed. [0005] More in particular, in the case of an acid solution that also contains metal cations, the process according to WO/2018/142349 provides supplying an amount of ammonia high enough to alkalinize the solution and to precipitate the cations as separable hydroxides, in order to crystallize the ammonium salt product, because the addition of ammonia can cause water to be removed from the mixture only in an alkaline environment. In any case, a concentrated solution of the acid cannot be obtained by this process.

[0006] US 6056880 relates to a thermal process of sterilizing a waste sludge. The process is an improvement with respect to the traditional lime treatments, which do not make it possible to increase the temperature of the sludge to a value high enough to sterilize them and to eliminate specific patho genic agents. The process includes introducing the sludge into a duct and there mixing it with an acid, preferably sulfamic acid and, subsequently or simultane ously, with oxides and/or hydroxides selected from the group consisting of Ca, Na, K, Li oxides/hydroxides. The oxides/hydroxides react exothermically with the acid, thus heating the mass. A magnetic heating device, or a fluid heating device arranged along the duct also contributes to the heating. A screw convey or is arranged within the duct for maintaining the sludge under pressure within the duct during at least 15 seconds. At the outlet of the duct the sludge quickly expands into an expansion chamber where it is collected and subsequently ex- tracted from the bottom, while a water-containing vapour exits through a bleed valve connected to the treatment duct. As a less advantageous alternative to sulfamic acid, US 6056880 also mentions carbonic acid, which can be ob tained in situ by causing the CO2 to pass through the sludge, the CO2 combining with the water contained in the sludge. [0007] US 2005/109713 relates to a process for removing interstitial water from a civil or industrial wastewater sludge. The interstitial water, retained within the cell membranes that are present in the sludge, is difficult to remove by such a conventional equipment as a filter press or a web filter. By these filters, a sludge is obtained that is too wet to allow correct operation of aerobic or anaer- obic digestion devices to which the sludge is normally sent. The process de scribed in US 2005/109713 includes pumping the sludge into a duct where gas eous CO2 is also injected under pressure, in order to saturate the sludge. The sludge treated this way is allowed to expand into a reservoir through an outlet orifice of the duct, in order to release the CO2. CO2 is known for being able to cross the cell membranes and to dissolve into the interstitial water that is pre sent within them. By flashing at the outlet of the duct, the CO2 causes the mem branes to break and the interstitial water to be released into the mass of the sludge. The released CO2 can be collected in view of a possible reuse in the process, while the treated sludge can be filtered in a conventional filter until a minimum moisture content is left, and can therefore be sent to a biologic stabili zation treatment. This treatment is also assisted by the smaller size of the or ganic particles and by their consequently higher surface/volume ratio. Before being flashed, the sludge saturated with CO2 can be acidified in order to pro- mote the release of the CO2 and the destruction of pathogenic organisms.

[0008] US 5 122350 describes a process for making magnesium-rich mixed calcium and magnesium acetate starting from dolomite. A step is provided of calcinating the dolomite at 550-650°C, during which CO2 is introduced to selec tively decompose the magnesium carbonate and not the calcium carbonate, thus obtaining magnesium oxide and CO2. The product of the calcination is al lowed to ferment in acetic acid (at a slightly acid pH) at 50-60°C and under an anaerobic atmosphere that contains CO2 along with CO or H2. A separation is then performed of the Ca / Mg acetate solution obtained from a calcium-rich sol id residue, followed by an evaporative concentration of the solution along with a possible crystallization of the acetate.

Summary of the invention

[0009] It is therefore a feature of the present invention to provide an alterna tive process for removing water from a mixture that does not provide steps of thermal evaporation of the water, and that accordingly does not involve the high energy consumption thet is associated with the evaporation and allows using low-enthalpy heat sources.

[0010] It is also a feature of the invention to provide such a process for treat ing acid mixtures, in particular a process for concentrating solutions of mineral or organic acids, and for treating an acid waste water also containing metal cati ons in a way much easier than the prior art.

[0011] It is also a feature of the invention to provide such a process that can be safely used to concentrate food products.

[0012] These and other objects are achieved by a process as defined by at tached claim 1 . Exemplary specific embodiments of the invention are defined by the dependent claims.

[0013] According to the invention, in order to remove water from a mixture containing water and a compound, the mixture having a pH value lower than or equal to 7, and being at an initial mass ratio between the water and this com pound, comprises the steps of: prearranging a treatment container; defining a target mass ratio between said water and said compound, said target mass ratio lower than said initial mass ratio; feeding the mixture into the treatment container; feeding a stream of a treatment gas containing carbon dioxide CO2 into the treatment container and causing the treatment gas to contact the mixture, in such a way that a part of the carbon dioxide reacts with the water of the mixture, forming carbonic acid H2CO3; maintaining the mixture at a predetermined treatment temperature; extracting an exhaust gas stream from the treatment container, said ex haust gas comprising water and carbon dioxide, so as to progressively remove water from the mixture while the treatment gas stream is supplied and the exhaust gas stream is extracted into/from the treatment container and, therefore, so as to concentrate the mixture; wherein a step is provided of: determining a current mass ratio between the water and the compound in the mixture; comparing the current mass ratio with the target mass ratio; if the current mass ratio is lower than the target mass ratio, continuing the steps of feeding, maintaining and extracting, repeating the steps of deter mining and comparing; until a final concentrate of said compound is obtained having the prede fined final ratio between the water and the compound: withdrawing the concentrate as a final concentrate from said treatment container.

[0014] By bubbling the treatment gas containing CO2 through the liquid mix ture, a part of the gas tends to dissolve into the water and reaches, at least at the interface between the gas phase and the liquid phase, an equilibrium be tween the carbon dioxide C02(g) in the gaseous state and the carbon dioxide C02(aq) in a molecularly solvated state:

[1] C0 ₂ (g) ^ CO2 (aq),

H = [C0 ₂ ]/PCO ₂ = 3.4-10- ² mol/ -atm), while carbonic acid H2CO3, which cannot be isolated and can only exist in the solution, is formed in the liquid phase:

[2] C0 ₂ (aq) + H2O (I) _< ® H2CO3 (aq),

K = [CO2HH2O H2CO3] = 1 .7-1 O ³ .

The chemical equations [1] and [2] contain the liquid-phase molar concentra- tions [CO2], [H2O], [H2CO3] and the gas-phase partial pressure Pco ₂ of carbon dioxide. At a room temperature and pressure, the formation rate of H2CO3 is very slow, therefore most of the CO2 remains in the solvated state.

[0015] By continuing the feeding of the treatment gas, the equilibrium equa tion [2] is shifted towards the formation of H2CO3, causing further CO2 to pass into the liquid phase according to the equation [1] An excess carbon dioxide is supplied with respect to the stoichiometric amount corresponding to the amount of water that must be removed, established according to the equation [2], there fore a part of the treatment gas stream acts as a carrier gas, removing from the system a vapour phase tendentially in equilibrium with the liquid. [0016] Therefore, the system will tend to evolve as a succession of equilibri um conditions at the interface, so further CO2 and water molecules gradually pass from the liquid phase to the vapour phase, and then are withdrawn along with the treatment gas stream, which works as a carrier gas. This mechanism continues causing the amount of water in the mixture contained in the treatment container to progressively decrease, until the predefined target mass ratio be tween the water and the compound is attained, i.e. until a desired concentration of the compound or a desired crystallization / drying grade of the compound, in the solid state, is reached, depending on the application for which the process is used.

[0017] Actually, the liquid phase contains water H20(l) in the liquid state, carbon dioxide C02(aq) in the solvated state and carbonic acid H2C03(aq). Each of these components, in the liquid phase, follows a liquid-vapour equilibri um law, i.e. it will tend to an equilibrium condition with the same component in the vapour phase. However, the carbonic acid H2CO3 can exist only in the liquid phase, and not in the vapour phase, therefore, when passing to the vapour phase, it will “give back” the amount of CO2 and water that have reacted accord ing to the reaction [2] By dynamically perturbing the succession of equilibria of [1] CO2 dissolution into water, [2] reaction between water and CO2 to form car bonic acid, and phase change to vapour phase with decomposition of the car bonic acid forming CO2 and H2O gas/vapour, a net result of removing water from the treated liquid mixture is obtained.

[0018] The process, i.e. the steps of supplying carbon dioxide, maintaining the treatment temperature and extracting the exhaust gas, is/are preferably car ried out at a pressure close to atmospheric pressure, even if it would be possi ble to operate under pressure, since a higher pressure would shift the equilibri um reaction [2] towards the right-hand side. Unlike the thermal evaporation and stripping processes, no substantial advantages would be involved by operating under vacuum. However, a slight vacuum degree will be established due to the equipment required for removing the exhaust gas and for discharging it, for in stance, into an atmospheric pressure environment. Therefore, the process is preferably carried out at a pressure higher than -0.20 relative bar, more prefera bly at a pressure higher than -0.10 relative bar, even more preferably at a pres sure higher than -0.05 relative bar.

[0019] The process according to the invention is well-suited to treat acid mixtures, in particular in order to concentrate any acid solution, i.e. solutions of mineral and/or organic acids. In fact, in these conditions, the carbonic acid dis sociates into bicarbonate and/or carbonate ions, so the formation of respective insoluble salts involving metal cations that are possibly present in the mixture is hindered at low pH values. It is worthwhile to remind that most industrial waste waters consist of an acid mixture also containing several kinds of cations. [0020] According to the above, the process makes it possible to directly treat acid solutions by the treatment gas, without any previous alkalinization and, in particular, it makes it possible to obtain acid concentrated solutions starting from diluted solutions. On the contrary, the prior art process (WO/2018/142349) is not suitable for concentrating acid aqueous solution, since it leads to the concentra tion of ammonium salts that are formed due to the use of ammonia. Therefore, the application field of the process according to WO/2018/142349 is different from that of the present invention.

[0021] In the light of the above, preferably, the pH of the mixture at the be- ginning of the treatment is lower than 5, even more preferably it is lower than 3. These acid mixtures can be directly treated with a C02-containing treatment gas.

[0022] In particular, the above acid is sulphuric acid, and the treatment tem perature is increased as sulphuric acid concentration increases. More in particu- lar, the treatment temperature is set between 45°C and 75°C until a first sul phuric acid concentration in the mixture lower than 85% is reached, and is pro gressively increased up to a temperature lower than the boiling temperature of the mixture until a second sulphuric acid concentration between 85% and 98% is reached, for instance the treatment temperature is increased up to 190°C. [0023] In particular, the process according to the invention makes it possible to concentrate solutions of inorganic acids, such as hydrochloric acid, sulphuric acid, nitric acid, phosphoric acids, boric acid, arsenic acid, antimonic acid, hy- pochlorous acid, chlorous acid, chloric acid, perchloric acid, hypobromous acid, bromous acid, bromic acid, perbromic acid, hypoiodous acid, iodous acid, iodic acid, periodic acid, and the like; solutions of organic acids, for example oxalic acid, formic acid, acetic acid, propionic acid, citric acid, malic acid, tartaric acid, sorbic acid, lactic acid, ascorbic acid, and the like.

[0024] Advantageously, the total amount of carbon dioxide supplied during said step of continuing the steps of feeding carbon dioxide, maintaining the treatment temperature and extracting the exhaust gas is at least 10% higher than said stoichiometric amount, preferably this amount of carbon dioxide is at least 18% higher than the stoichiometric amount, more preferably it is at least 25% higher than the stoichiometric amount, even more preferably it is at least 30% higher than the stoichiometric amount. [0025] In a semicontinuous mode of the process, in which an amount of the mixture to be treated is preliminarily introduced into the treatment container, the step of continuing lasts for a given treatment time, and the step of withdrawing the concentrate takes place once that treatment time has elapsed, the amount of supplied carbon dioxide can be predetermined choosing an appropriate com bination of the treatment time, of the concentration of carbon dioxide in the treatment gas, and of the treatment gas flowrate.

[0026] In a continuous mode of the process, in which the step of feeding the mixture to be treated, the continuation step and the step of withdrawing the con centrate are carried out simultaneously, the amount of supplied carbon dioxide can be predetermined by selecting an appropriate combination of the carbon di oxide concentration, of the treatment gas flowrate, and of the residence time of the mixture in the treatment container, which can be established according to the size of the treatment container, which can be a tubular container or a mixed vessel.

[0027] Advantageously, the concentration of the CO2 in the treatment gas is higher than 50%. In fact, the inventors have found that a remarkably higher mix ture concentration rate is obtained by a concentration above this value. If a mix ture air/carbon dioxide is used as the treatment gas, this is probably due to the lower density of air with respect to carbon dioxide.

[0028] Preferably, the CO2 concentration in the treatment gas is higher than 70%%, more preferably the treatment gas comprises substantially or technically pure CO2. Actually, in order to assist the step of feeding the treatment gas, it is preferable to use the same CO2 stream for both establishing the chemical equi librium according to the equation [2], and using the stream as a carrier gas stream, which allows for an industrially acceptable vapour phase removal rate. [0029] Advantageously, the treatment temperature is higher than 30°C, preferably it is higher than 40°C, even more preferably it is set between 45°C and 75°C. In these conditions and, with respect to the methods based on ther mal evaporation of water: a heat source can be exploited even if it has a remarkably lower tempera ture, since it is not necessary that the heat source is available at a temper ature higher than the boiling point of the water: this makes it possible to exploit the residual heat of many production processes, while allowing for an acceptable process rate; a lower amount of heat is required, since the heat transferred to the solu tion must substantially only compensate the heat loss from the treatment container, which are low due to the low treatment temperature. In fact, both the reaction of formation of H2CO3 and the reverse reaction are substan tially athermic, and in any case they do not require heat. On the contrary, in an evaporation process, the high evaporation enthalpy of the water must be accounted for, unless the evaporation is carried out under high vacuum. For instance, the reverse reaction can take place in a condenser, in an embodiment of the process in which steps are provided of condensing and collecting the water present in the exhaust gas, as described hereinafter. [0030] Moreover, the low temperature at which the water removal is carried out makes it possible to concentrate mixtures in which thermolabile components are present, such as vegetable juices like fruit juices or veggie juices. In this case, the process of the invention is therefore further preferable with respect to any thermal concentration process, since it does not require vacuum conditions, and the related plant and operation costs are not involved. On the contrary, vacuum conditions are required in the thermal concentration processes, in order to limit the treatment temperature within the limits allowed by the stability of the thermolabile components.

[0031] As already described, the process is suitable for treating a non- homogeneous mixture. The compound can be dissolved in the water, forming a homogeneous solution, or can be present in the mixture as a solid, in the form of a suspension or of a dispersion, since the beginning or since an intermediate step of the process.

[0032] The process according to the invention can also be used to concen trate salts involving either acid or neutral hydrolysis, which are generally salts of strong acids, such as sulphates, chlorides, phosphates, nitrates, iodates and the like.

[0033] In particular, the process according to the invention can be used in a crystallization process of a salt having a saturation concentration in water, in which case the mixture is initially homogeneous; in this case, the steps of feed- ing, maintaining and extracting the exhaust gas are continued until a target mass ratio is attained corresponding to the saturation concentration, so as to crystallize the salt.

[0034] The mixture can also be a two-phase mixture, in which case a precip- itate of the compound is present, and the mixture has the form of a slurry, for in stance a brine, or in particular it has the form of a sludge, in which the liquid phase is the dispersed phase and the solid phase is the continuous phase. The mixture can also be a two-phase mixture already at the beginning the treatment, or only at a late step thereof. In other words, in a crystallization and/or drying process, during the treatment, a phase inversion may take place, in which the mixture becomes a wet material that contains imbibition water or, in the case of a salt, a wet material that can also contain hydration water. To this purpose, the steps of feeding, maintaining and extracting the exhaust gas stream can be con tinued until a target mass ratio is attained between the water and the compound that corresponds to a predetermined residual amount of moisture per mass unit of the compound, in particular until the compound is obtained in a substantially dry status or, in particular, in an anhydrous state, in the case of a salt that can have at least one hydration state with a given number of hydration water mole cules. [0035] Preferably, a final step is provided of removing a carbon dioxide resi due from the final concentrate, including a step selected from the group consist ing of: creating mild vacuum conditions; slightly heating up to a temperature high enough to cause a removal of carbon dioxide corresponding to a predetermined purification degree; stirring the concentrate; a combination thereof.

[0036] In an exemplary embodiment, the treatment container is a reservoir, the mixture forming a liquid head in the reservoir, and the carbon dioxide- containing treatment gas stream is supplied below this liquid head.

[0037] In an exemplary embodiment of the process, the mixture contains a wet solid comprising said compound or a plurality of compounds, in particular, the mixture can be a waste sludge from an industrial process, and the water is an imbibition water of the solid. Preferably, in this case, a preliminary step is provides of aggregating the sludge into particles having a predetermined parti cle-size distribution, in order to promote the contact with the treatment gas. [0038] Preferably, steps are provided of condensing the water from the ex haust gas stream, and of collecting the condensed water. A step can also be provided of conveying back to the treatment container the CO2 that has been separated from the water as an incondensable gas.

[0039] The condensed water contains a normally small amount of CO2, ac cording to the solubility equilibrium of the equation [1] and depending therefore on the temperature attained during the condensation. This CO2 amount can be withdrawn by a conventional technique such as mild stirring and/or creating vacuum conditions, in order to be re-used in the process, along with the CO2 separated from the exhaust gas as an incondensable gas before the condensa tion step.

Brief description of the drawings [0040] The invention will be now shown with the following description of its exemplary embodiments, exemplifying but not limitative, with reference to the attached drawings, in which:

Fig. 1 shows a block diagram of the process according to the invention;

Fig. 2 shows a block diagram of the process, according to exemplary embodiments in which steps are provided of condensing and collecting the water and of recycling the CO2 to the treatment container, respectively; Figs. 3 and 4 show a flow-sheet of an apparatus for carrying out the process of Fig. 1 during the treatment and at the end of it, respectively;

Fig. 5 shows a flow-sheet of an apparatus for carrying out the process according to the embodiments to which Fig. 2 relates.

Description of preferred exemplary embodiments

[0041] With reference to Figs. 1 , 3 and 4, a process is described for remov ing water from a mixture 2 containing at least one compound, in addition to wa ter. [0042] The process comprises a preliminary step 101 of defining a target mass ratio Fit between the mass of the water that is present in the mixture and the mass of the compound that is present in mixture. Such mass ratio is just one of the possible parameters that can be considered as parameters related to the final water content that must be obtained by the process. For instance, the mass ratio between the compound or the water and the total mass of the mixture can also be considered. As an alternative, volume ratios between the water and the compound can be considered, as well as volume ratios between the water or the compound and the mixture, or mixed mass/volume or volume/mass ratios. The mass and the mass ratios can be expressed both in mole and in weight units, depending on the specific case.

[0043] The process also comprises a preliminary step 102 of prearranging a treatment container 11 , preferably a vessel, containing mixture 2 that must be treated. After prearranging step 102 a step 110 takes place of removing the wa ter from mixture 2, which comprises the steps 111 , 112 and 113 described here inafter.

[0044] Firstly, a step 111 is provided of feeding a stream of a treatment gas 3 containing carbon dioxide into treatment container 11. Step 111 of feeding treatment gas 3 can be carried out in a conventional way, for instance, in the case of a globally liquid mixture 2, through a duct 13 arranged in the container so as to be submerged by the liquid, the duct being provided with nozzles or other distribution devices, not shown. Feeding 111 is carried out in such a way to promote the contact of treatment gas 3 with the water of mixture 2, in order to cause the water to react with the carbon dioxide CO2 according to the reaction

C0 ₂ (g) + H2O (I) ¾ H2CO3 (I) [2] forming carbonic acid FI2CO3. If mixture 2 comprises a wet solid, a preliminary step of granulating the mixture is advantageously provided, i.e. a step of aggre gating it into particles having a predetermined particle size distribution, in order to assist this contact.

[0045] A step 112 can also be provided of heating and then maintaining mix ture 2 to/at a treatment temperature T. To this purpose, a temperature control device can be provided for container 11 , including a jacket 15 or any equivalent means. Advantageously, the treatment temperature is higher than 30°C. In fact, a treatment temperature T even slightly higher than this value ensures that the reaction [2] takes place. In order to obtain an industrially more favourable pro cess rate, treatment temperature T is preferably equal to or higher than 40°C. In order to maintain this temperature, it is sufficient to supply temperature control device 15 with a heating fluid 16 heated or formed by a low-enthalpy heat source, not shown, for example a heating fluid that has been already used for another heating or temperature control operation, which can be easily found in an industrial plant. For instance, heating fluid 16 can be condensation water, or vapour at a pressure slightly higher than atmospheric pressure, or tempered wa ter generated therefrom. Heating fluid 16 can be returned to the heat source as an exhaust heating fluid 16’, said fluid being ready for performing a new heating cycle and generating heating fluid 16.

[0046] In these conditions, a step 113 takes place of extracting a stream of an exhaust gas mixture 4 from treatment container 11. Exhaust gas mixture 4 comprises a part of treatment gas 3 that has not reacted with the water of mix ture 2, i.e. unreacted carbon dioxide, or any possible gas present in stream 3, in addition to vapour water.

[0047] As described, step 110 of removing the water, i.e. steps of feeding 111 treatment gas 3, maintaining 112 treatment temperature T and extracting

113 exhaust gas and vapour 4, is/are continued until, by removing water through the reaction [2], the content of water of mixture 2 has become lower than a predetermined or target value, for instance, until the mass ratio between the water and the compound of mixture 2, expressed in weight or mole units, has reached a predetermined final or target value, lower than the starting value, obtaining this way a final concentrate 2’ of the compound in water (Fig. 4).

[0048] In order to check whether this object has been attained, a step 121 is provided of determining a current mass ratio R between the water and the com pound present in mixture 2, or an equivalent weight or volume or mixed ratio, as previously discussed, and also a step 125 is provided of comparing the value of this current mass ratio R, or of any equivalent current amount ratio, and the val ue of target mass ratio Rt or the value corresponding to the target condition of a different amount ratio that is coherent with the determined current ratio.

[0049] In a way equivalent to the above, comparison step 125 can be carried out by comparing current amount values and target amount values of the water or of mixture 2. For example, these amount values can comprise the mass of mixture 2, in which case the current value can be determined, for instance, by weighing treatment container 11 , because the target or final value that the mix ture will have upon reaching the target conditions is known; the volume of mix- ture 2, in which case the current value can be determined by measuring the lev el of mixture 2 that is present within treatment container 11 ; the mass or the vol ume of the water extracted from treatment container 11 along with exhaust gas mixture 4, in which case the current value can be determined, for instance, by weighing or reading the volume of the water that has been condensed in an ex changer 30 and has been collected, for instance, in a reservoir 40, as described hereinafter with reference to Fig. 5, or it can be determined by measuring the amount of exhaust gas mixture 4 extracted from treatment container 11 and the amount of treatment gas 3 supplied to treatment container 11 , and then compu- ting the difference between these amounts.

[0050] In any case, if step 125 of comparing indicates that the content of wa ter accompanying the compound is higher than the predetermined value, i.e. if, for instance, the value of the current mass ratio between the water and the compound is higher than the value of the target mass ratio between the water and the compound, step 110 of removing the water is continued, i.e. the steps of feeding 111 treatment gas 3, maintaining 112 treatment temperature T and ex tracting 113 exhaust gas and vapour 4 are continued, as indicated above, re peating steps 121 of determining said current mass ratio, or the current value of an equivalent ratio, and 125 of comparing this current value with the target value predefined in step 101.

[0051] In the case of a semicontinuous mode of the process, as defined above, steps 121 of determining and 125 of comparing can be carried out by fol lowing a predetermined time program during step 110 of removing the water, for example a program according to which these steps are performed at decreas- ing-length time intervals, i.e. at times progressively closer to one another as the target value of water residual mixture is more likely to be attained, as it can be established by a skilled person’s experience. As an alternative, steps 121 of de termining and 125 of comparing can be carried out by an operator once a prede termined time has elapsed since the beginning of step 110 of removing the wa- ter.

[0052] In the case of a continuous mode of the process, as defined above, steps 121 of determining and 125 of comparing can be performed in line, i.e. along a flow of the treated mixture, for example at the exit of a treatment con tainer, in particular a tubular treatment container, and the treated mixture is con- veyed back to the inlet of the container itself, for example through a treatment container feed tank, as long as, or in any case if step 125 of comparing indi cates that the content of water accompanying the compound is higher than the target value, and the treated mixture is extracted when step 125 of comparing indicates that the content of water accompanying the compound is lower than or equal to the target value.

[0053] Mixture 2 can be a homogeneous mixture, for example a homogene ous solution of a salt, or a heterogeneous liquid mixture, in which the compound is present as a dispersed or suspended solid, including a slurry. In particular, the compound can be a salt at a concentration higher than the saturation con centration, or a water-insoluble solid.

[0054] In the case of a homogeneous solution 2 in which a salt that has a mass fraction or concentration lower than the saturation value, extraction 113, i.e. removal 110 of water from solution 2, can be continued until the concentra- tion exceeds the saturation value of the salt, causing the salt to precipitate. This is the case of a crystallization process, in which the process according to the in vention is advantageously used, and in which a step can follow of mechanically separating, for example by filtering, the crystallized salt from the water of con centrate 2’ that has been obtained from solution 2, i.e. from the mother liquor. [0055] As an alternative, removal 110 of the water can be continued until the mother liquor is finished, so that the wet crystallized solid remains in treatment container 11 , and the liquid phase is present only as imbibition water. If water removal step 110 is further continued, water can be finally present substantially only as hydration water of the salt, obtained in this case as a possible hydrated form, or in particular the target mass ratio between water and solute can be ze ro, which corresponding to a dry salt or solid.

[0056] Once removal 110 of the water from mixture 2-2’ has been complet ed, a step 115 can optionally be provided of removing a carbon dioxide residue from obtained final concentrate 2’. Removal 115 can comprise for example a treatment using a stream of a gas, such as air. As an alternative, or in addition to the above, a step can be provided of creating predetermined vacuum condi tions. As an alternative, or in addition to the above, a step can be provided of heating up to a predetermined purification temperature, for example up to incipi- ent boiling of concentrate 2’, when the latter is a liquid. As an alternative, or in addition to the above, a step can be provided of stirring concentrate 2’.

[0057] A last step is provided 199 of withdrawing final concentrate 2’, by conventional discharge means 19. [0058] Mixture 2 can also have the consistency of a wet solid, not necessari ly a salt, even since the beginning of the process. For instance, mixture 2 can be an industrial sludge to be treated, in which the water is present as imbibition water and is a dispersed phase. As an alternative, mixture 2 can be a true solid, for instance it can be a compound as a salt, in which the water is present as hy- dration water. In these cases, removal 110 of the water is continued until the amount of moisture in the sludge is reduced to a predetermined residual amount. In an extreme condition, the sludge can be turned into a substantially dry matter.

[0059] With reference to Figs. 2 and 5, a process is described according to an exemplary embodiment of the invention, in which also a step 130 is provided of treating the stream of exhaust gas and vapour 4. Treatment 130 includes a step 131 of condensing the water contained in the stream of exhaust gas and vapour 4, and a step 132 of collecting the water 5 obtained this way.

[0060] Condensation 131 of the water can be carried out in an apparatus 30 of known type such as a tube-and shell exchanger 30 connected to a refrigerant fluid supply system 36. Condenser 30 is advantageously configured for treating an incondensable gas portion, including the excess carbon dioxide contained in treatment gas 3. A corresponding stream 4’ of incondensable gas is preferably separated from condensed water 5 downstream of condenser 30, and is con- veyed back to treatment container 11 as treatment gas 3, possibly after adding a make-up stream 3’ in order to compensate the loss of carbon dioxide due to the reaction, or due to physical absorption into water 5. A fan 70 is advanta geously used to convey incondensable gas stream 4’ back to treatment contain er 11. [0061] Collection 132 of the condensed water is preferably carried out in a collection reservoir 40 of known type, including a vent nozzle connected to a treatment device 41 and an outlet nozzle 42 for collected condensed water 5. [0062] A step 135 can optionally be provided of removing a carbon dioxide residue from condensed water 5, including such conventional operations as heating, stirring, treating by a gas stream and creating vacuum conditions, or a combination thereof.

[0063] The present description essentially relates to a semicontinuous mode of the process, as defined above. However, the process can be carried out also in a continuous mode, as defined above.

EXAMPLES

Example 1 : Concentration of an aqueous solution of sulphuric acid from

15% to 85%

[0064] 100 Kg of a 15% by weight aqueous solution of sulphuric acid

H2SO4 were put into a treatment container. A stream of pure carbon dioxide treatment gas was bubbled into the same container, while the treatment tem perature of the mixture was maintained at 50°C.

[0065] The carbon dioxide introduced into the treatment container shifted the equilibrium reaction

C0 ₂ (g) + H2O (I) ¾ H2CO3 (I) [2] towards the right-hand side, i.e. towards the formation of H2CO3, and also pro moted the extraction of the equilibrium vapour phase, i.e. of an exhaust gas stream, from the treatment container, said vapour phase formed starting from the treatment gas and containing vapour water. This way, carbon dioxide and water molecules passed into the vapour phase, therefore the water was pro gressively withdrawn from the aqueous solution present in the treatment con tainer, thus progressively increasing the sulphuric acid concentration H2SO4. [0066] The vapour was condensed, and the condensate was collected in a collection reservoir, while the excess carbon dioxide, which could not conden sate, was recycled to the treatment container, in order to limit the amount of consumed carbon dioxide.

[0067] After about 4 and a half hours, 17.6 Kg of an aqueous solution of sulphuric acid at the target 85% concentration were left in the treatment con tainer, substantially free from residual dissolved CO2, while about 82.4 Kg of wa ter were collected in the condensate collection reservoir. The carbon dioxide was fed at a 60 Kg/hr flowrate, which corresponds to about 34% excess with re spect to the stoichiometric amount required to remove such an amount of water, according to equation [2] Example 2: Concentration of an aqueous solution of sulphuric acid from

85% to 98%

[0068] The test of Example 1 was continued in order to further concentrate the sulphuric acid up to 98%. To this purpose, the treatment temperature of the solution in the treatment container was raised up to 190 ^Q C, and was maintained below the boiling temperature of the mixture at the various concentration of the same, still bubbling the carbon dioxide at the same flow rate. About 2 hours lat er, 15.3 Kg of an aqueous solution of sulphuric acid at the target concentration of 98% were left in the treatment container, while about 2.3 Kg of water were collected in the condensate collection reservoir.

Example 3: Concentration of a solution of magnesium sulphate from 15% to 25%

[0069] 150 Kg of a 15% by weight aqueous solution of Magnesium sul phate MgSC>4 were put into a treatment container. A stream of pure carbon diox ide treatment gas was bubbled into the same container, while the treatment temperature of the mixture was maintained at 50°C.

[0070] The carbon dioxide introduced into the treatment container shifted the equilibrium reaction [2] towards the right-hand side, and also assisted the extraction of the equilibrium vapour phase, i.e. of an exhaust gas stream, from the treatment container, said vapour phase formed starting from the treatment gas and containing vapour water. This way, carbon dioxide and water molecules passed into the vapour phase, therefore the water was progressively withdrawn from the aqueous solution present in the treatment container, thus progressively increasing the sulphuric acid Mg2SC>4 concentration.

[0071] The vapour was condensed, and the condensate and the inconden- sable excess carbon dioxide were treated as described in the example 1.

[0072] After about 3 hours, 90 Kg of an aqueous solution of magnesium sulphate at the target concentration of 25% were left in the treatment container, substantially free from residual dissolved CO2, while about 60 Kg of water were collected in the condensate collection reservoir. The carbon dioxide was fed at a 65 Kg/hr flowrate, which corresponds to about 33% excess with respect to the stoichiometric amount required to remove such an amount of water, according to equation [2] Example 4: Concentration of sea water and crystallization of the salt.

[0073] 100 Kg of sea water with a salt content of 3.5% by weight were put into a treatment container. A stream of pure carbon dioxide treatment gas was bubbled into the same container, while the treatment temperature of the mixture was maintained at 50°C.

[0074] The carbon dioxide introduced into the treatment container shifted the equilibrium reaction [2] towards the right-hand side, promoting the formation of H2CO3 and also assisting the extraction of the equilibrium vapour phase, i.e. of an exhaust gas stream, from the treatment container, said vapour phase formed start- ing from the treatment gas and containing vapour water. This way, carbon dioxide and water molecules passed into the vapour phase, therefore the water was pro gressively withdrawn from the aqueous solution present in the treatment container, thus progressively increasing the salt concentration in the treatment container. [0075] The vapour was condensed, and the condensate and the inconden- sable excess carbon dioxide were treated as described in the example 1 .

[0076] After about 5 hours, 3.5 Kg of salt were left in the treatment contain er, while about 96.5 Kg of water were collected in the condensate collection reservoir. The carbon dioxide was fed at a 60 Kg/hr flowrate, which corresponds to about 27% excess with respect to the stoichiometric amount required to re- move such an amount of water, according to equation [2]

[0077] The CO2 (g) dissolved in the water was easily withdrawn by heating and treating with air, in order to be used as drinkable water, after a mineraliza tion treatment.

Example 5: Elimination of the hydration water from a pentahvdrate salt (Copper sulphate CuS04.5H20)

[0078] 125 Kg of a saturated solution of copper sulphate were put into a treatment container. The solution had been previously prepared from 30 Kg of pentahydrate salt CUSO4.5H2O and 95 Kg water. A stream of pure carbon diox ide treatment gas was bubbled into the same container, while the treatment temperature of the mixture was maintained at 75°C.

[0079] The carbon dioxide introduced into the treatment container shifted the equilibrium reaction [2] towards the right-hand side, promoting the formation of H2CO3, and also assisting the extraction of the equilibrium vapour phase, i.e. of an exhaust gas stream, from the treatment container, said vapour phase formed starting from the treatment gas and containing vapour water. This way, carbon dioxide and water molecules passed into the vapour phase, therefore the water was progressively withdrawn from the aqueous solution present in the treatment container, thus progressively increasing the salt concentration in the treatment container.

[0080] The vapour was condensed, and the condensate and the inconden sable excess carbon dioxide were treated as described in the example 1.

[0081] After about 6 hours, 19 Kg of dry anhydrous salt, i.e. of CuSC with- out hydration water were left in the treatment container, while about 106 Kg of water were collected in the condensate collection reservoir. The carbon dioxide was fed at a flowrate of 55 Kg/hr, which corresponds to about 27% excess with respect to the stoichiometric amount required to remove such an amount of wa ter, according to equation [2] [0082] The CO2 (g) dissolved in the water was easily withdrawn by mild heating and stirring.

Example 6: Concentration of fruit juice from 12% to 70%.

[0083] Into a treatment container were arranged 100 Kg of orange juice with a 12% by weight concentration of dissolved solids, mainly sugars. A stream of pure carbon dioxide treatment gas was bubbled into the same container, while the treatment temperature of the mixture was maintained at 40°C.

The carbon dioxide introduced into the treatment container shifted the equilibri um reaction [2] towards the right-hand side, promoting the formation of H2CO3, and also assisting the extraction of the equilibrium vapour phase, i.e. of an ex- haust gas stream, from the treatment container, said vapour phase formed start ing from the treatment gas and containing vapour water. This way, carbon diox ide and water molecules passed into the vapour phase, therefore the water was progressively withdrawn from the aqueous solution present in the treatment con tainer, thus progressively increasing the solid concentration in the orange juice. [0084] The vapour was condensed, and the condensate and the inconden sable excess carbon dioxide were treated as described in the example 1.

[0085] After about 4 hours, 17 Kg of an aqueous solution of orange juice concentrated to 70% were left in the treatment container, while about 83 Kg of water were collected in the condensate collection reservoir. The carbon dioxide was fed at a flowrate of 67 Kg/hr, which corresponds to about 32% excess with respect to the stoichiometric amount required to remove such an amount of wa ter, according to equation [2] [0086] The residual carbon dioxide in the treatment container was removed as described in the example 1 under mild vacuum conditions.

[0087] The foregoing description of exemplary embodiments and specific examples of the invention will so fully reveal the invention according to the con ceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such embodiments and specific examples without further research and without parting from the invention, and, accordingly, it is therefore to be understood that such adaptations and modifica tions will have to be considered as equivalent to the specific embodiment and of the examples. The means and the materials to put into practice the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseol ogy or terminology employed herein is for the purpose of description and not of limitation.