Abstract:
A multi-stage counter-current crystallization apparatus for crystallizing and subsequently separating a component in pure form from a solution comprises a first concentration stage for forming crystals in the solution and a first crystallization vessel for further growth of the crystals to obtain a first crystal suspension mixture containing larger crystals and first separating means for separating the larger crystals from the first crystal suspension mixture to form a concentrated mother liquor. A second concentration stage forms crystals in the concentrated mother liquor solution and includes a second crystallization vessel for further growth of the crystals. A bypass conduit circulates the crystal suspension mixture in a crystal-growing vessel, and includes a filter for extracting part of the mother liquor solution from the crystal suspension mixture.

Description:
FIELD OF THE INVENTION 
     The invention relates to a multi-stage counter-current crystallization apparatus for crystallizing and subsequently separating a component in pure form from a solution. 
     BACKGROUND OF THE INVENTION 
     An apparatus for separating a component from a solution, also referred to by the common term suspension crystallization apparatus, is used both in the foodstuffs industry and in the chemical/petrochemical processing industry. 
     In the foodstuffs industry, the solution is usually an aqueous solution, such as milk or fruit juices, in which water is the component to be separated and in which the final concentrate, for example milk concentrate, is processed further as an intermediate product. In the case of aqueous waste flows, water is likewise extracted from the solution by means of suspension crystallization, after which the concentrate that is finally obtained can simply be incinerated as a waste product. In the chemical/petrochemical industry this technique is used for obtaining a particular component in pure form from a chemical mixture, for example for the recovery of pure para-xylene from a mixture of isomers. 
     In such a crystallization apparatus, a slurry or suspension, also called crystal suspension mixture is formed, consisting of crystals (the component to be separated) and a concentrated solution, also called mother liquor solution. 
     A crystallization apparatus as described above is known, for example from European patent publication No. 0 051 340. An important parameter in industrial suspension crystallization processes is the crystal or ice fraction of the crystal suspension mixture that is present in large, agitated crystal-growing vessels. In order to produce (preferably large) crystals in the best possible way, which is important in connection with an optimum utilisation of the capacity of the separating means, a long residence time of the crystals in the crystal-growing vessel is required so as to obtain the highest possible crystal fraction in the crystal-growing vessel. With a view to obtaining an optimum and maximum yield of crystals, the ice crystals must be present in the crystal-growing vessel for a sufficiently long period of time, so as to enable them to grow into large ice crystals which are thus easy to separate. 
     Usually, such suspension crystallization apparatuses are composed of several concentration stages, so that the highest possible concentrated mother liquor is solution can be obtained in the final stage, whilst the separation of the crystals from the solution can be carried out at a lower concentration. The latter is important because the separation proceeds more easily as the concentration and thus the viscosity of the solution is lower. 
     In European patent publication No. 0 051 340 a filter is arranged in the crystal-growing vessel, so that only mother liquor solution is carried from the crystal-growing vessel to the crystallization means consisting of a heat exchanger of the scraped surface type. The object of this is to convert the small crystals from the heat exchanger into large, round crystals by allowing them to mature in the crystal-growing vessel rather than have the growth of said-crystals take place through circulation past the heat exchanger. Said filter, however, limits the maximally obtainable crystal fraction in the crystal-growing vessel. 
     The presence of the filter in the crystal-growing vessel does not permit a maximum crystal fraction, because this leads to accumulation of crystals and thus to clogging of the filter by the crystals and any other non-dissolved particles that are present. Since such a clogged filter cannot be cleaned, whether or not by means of complicated cleaning means, and must be kept free from crystals, during operation of the crystallization apparatus, a clogged filter in the crystal-growing vessel will inevitably lead to the apparatus being put out of operation, which is undesirable. 
     Since the filter in the crystal-growing vessel can also lead to a high pressure drop, the crystal-growing vessel must be in the form of a pressure vessel, which makes the purchase of the crystal-growing vessel in particular and indirectly also of the apparatus in its entirety a costly affair. In addition to that, the filters require a great deal of maintenance. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to obviate the drawbacks of the use of a filter in the crystal-growing vessel, to which end the crystal-growing vessel of at least one concentration stage is provided with a bypass conduit arranged outside the crystal-growing vessel for circulating the crystal suspension mixture that is present in the crystal-growing vessel, and wherein filter means are present in the bypass conduit for extracting at least part of the mother liquor solution from the crystal suspension mixture. 
     Surprisingly it has been demonstrated that although this arrangement according to the invention leads to more irregularly shaped crystals, this shape of the crystals does not have any negative effects on the capacity and effectiveness of the crystal separating means if the conditions are further identical. 
     More in particular, according to the invention the crystal-free mother liquor solution can function as the supply solution for a further concentration stage. 
     More in particular, the filter means consist of a pressure vessel arranged in the bypass conduit, which is provided with a mixing element and a filter, which filter can be cleaned at its crystal suspension side. The filter cleaning means may consist of a scraping element which Is movable relative to the filter surface. 
     In addition to that, a circulation pump may be arranged in the bypass conduit. 
     The invention furthermore relates to an apparatus for separating crystals from a crystal suspension mixture composed of crystals and mother liquor solution, comprising 
     a fully closed cylinder and a piston which is movable therein, which piston bounds a washing column chamber inside the cylinder; 
     at least one piston rod provided with a longitudinal bore, via which piston rod the piston is moved reciprocatingly in the cylinder by driving means; 
     filter means connected to the piston at the side of the washing column chamber; 
     a supply conduit, which is connected to the end of the piston rod that projects outside the cylinder, which conduit functions to supply the crystal suspension mixture to the washing column chamber through the longitudinal bore; 
     a discharge conduit for discharging mother liquor solution from the washing column chamber after filtration through the filter; 
     means arranged in the washing column chamber, near the closed side of the cylinder, for the controlled removal of an amount of crystals from the crystal bed that has been compacted in the washing column chamber by the piston; 
     a circulation conduit arranged outside the cylinder, which is in communication with the washing column chamber via a connection for the discharge of the removed amount of crystals and via a connection for the supply of a washing liquid to the compacted crystal bed; and 
     a conduit connected to the circulation conduit for discharging the removed crystals, which may or may not be in melted form. 
     Such a separating apparatus, which is also referred to as a washing column apparatus, is disclosed in, for examples U.S. Pat. No. 4,475,355. During operation, the crystal suspension mixture that is present in the washing column chamber is compressed by the piston, so that the crystals are compacted and carried to a circulation conduit by the removing means, for example a rotary disc fitted with scraping knives. 
     The circulation conduit, also referred to as the washing circuit, not only discharges the removed or scraped-off, compacted crystals from the washing column chamber, but it also supplies a washing liquid to the washing column chamber for washing the compacted crystal bed. Said washing liquid, in combination with a pressure exerted on the crystal bed by the piston, leads to the formation of an separation interface or wash front in the compacted crystal bed between the part of the compacted crystal bed near the removing means, in which the washing liquid has displaced the mother liquor solution that is present between the crystals, and the part of the compacted crystal bed near the piston or the filter means, in which the mother liquor solution is still present between the crystals. 
     The crystal bed that has been washed by the washing liquid can easily be carried off from the washing column apparatus with the aid of the removing means in order to obtain a pure component. The washing column apparatus thus effects the separation between the crystals and the mother liquor solution, which solution is carried to the crystal-growing vessel via the discharge conduit for further thickening or concentration. 
     According to the prior art, an adequate washing action requires the exertion of a high pressure by the piston on the compacted crystal bed that is present in the washing column chamber in order to build up a sufficiently high differential liquid pressure across the crystal bed, which leads to the desired flow of washing liquid towards the piston and the filter means for the purpose of displacing the mother liquor solution from the crystal bed. 
     Upon compacting of the crystal bed, the compacted crystal bed experiences a considerable amount of friction with the inner wall of the cylinder. In particular in the case of organic crystals and, to a lesser extent, ice crystals, said friction may become so high that further compacting of the bed by the piston is no longer possible. In such situations, the washing pressure to be built up by the piston is not high enough for displacing the mother liquor solution from the compacted crystal bed to a sufficient extent. 
     It is an object of the invention to obviate the aforesaid drawback and to provide a washing column apparatus by means of which a compacted bed of solid matter can be washed more efficiently. To this end, means for creating a differential liquid pressure across the compacted crystal bed during operation are arranged in the circulation conduit. 
     These additional pressure means extend the usability and the operating range of the washing column apparatus considerably, since the washing column can be operated at a higher washing pressure. Accordingly, the yield of solid matter increases considerably. 
     Specific embodiments of such a washing column apparatus are according to the invention characterized in that the pressure means consist of a restriction valve, which is incorporated in the circulation conduit between the discharge connection and the discharge conduit, or in that the pressure means consist of an external pump. 
     The pressure means may comprise an expansion vessel connected to the circulation conduit for creating the required differential liquid pressure by means of air pressure. 
     In another embodiment of the washing column apparatus according to the invention, the pressure means are controllable, more in particular by means of a sensor which detects the position of the separation interface between the part of the crystal bed in which the washing liquid has displaced the mother liquor solution that is present between the crystals and the part of the crystal bed in which the mother liquor solution is still present between the crystals. 
     In another embodiment, the supply conduit for supplying the crystal suspension mixture is connected to the longitudinal bore of the piston rod by means of a flexible intermediate conduit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be explained in more detail with reference to the drawings, in which: 
     FIG. 1 shows a first embodiment of the crystallization apparatus according to the invention; 
     FIG. 2 shows a preferred embodiment of a crystallization apparatus according to the invention; 
     FIG. 3 a  shows a first embodiment of a washing column apparatus according to the invention; and 
     FIG. 3 b  shows an enlarged part of the washing column apparatus according to FIG. 3 a;    
     FIG. 3 c  shows an enlarged part of another embodiment of a washing column apparatus according to the invention; and 
     FIG. 4 shows another embodiment of a washing column apparatus according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The crystallization apparatus as shown in FIG. 1 consists of two concentration stages  10  and  20 , respectively, which are each made up of a crystal-growing vessel  11 ,  21  provided with mixing means  11   a  and  21   a , respectively. A solution  13  is supplied to the crystal-growing vessel  11  from a supply vessel  1  and a supply conduit  13   a . Said solution may be an aqueous mother liquor solution, which mother liquor solution is to be thickened through suspension crystallization or cold crystallization. To this end, the mother liquor solution is drawn from the crystal-growing vessel  11  via the conduit  12  and a pump  12   a  and carried past a heat exchanger  12   b  of the scraped surface type. 
     The cooling down of the mother liquor solution at the heat exchanger  12   b  leads to the formation of crystals therein, which crystals can grow into large crystals as a result of a further decrease of the process temperature in the crystal-growing vessel  11 . Said growing process is enabled in part by mixing means  11   a , which keep the mother liquor solution in agitation, thus enabling the formation of crystals and preventing the formation of a thick lump and thus stagnation of the mixing means and the process apparatus. 
     Crystal suspension mixture (mother liquor solution plus crystals) can be drawn from the crystal-growing vessel  11  via the conduit  14   a  and be carried to a separating apparatus  15  or washing column. The crystals that have formed are separated from the concentrated mother liquor solution at that location in a manner yet to be described herein, which crystals are discharged via the conduit  15   b . The pure crystals that have formed will pass a heat exchanger  15   a  via conduit  15   b  upon leaving the washing column for the purpose of causing the removed crystals to melt in whole or in part. 
     The mother liquor solution, from which the crystals have been removed, is reintroduced into the crystal-growing vessel  11  via the conduit  14   b  for further concentration. By forming crystals anew in this circulating flow of concentrated mother liquor solution in the crystal-growing vessel  11  and separating them anew in the manner as described above, a further concentration of the solution  13  supplied via the supply vessel  1  can be obtained. 
     In order to obtain a further concentrated product, the apparatus according to the invention comprises a circulation conduit  16 , in which a circulation pump  16   a  is incorporated, by means of which pump crystal suspension mixture can be drawn from the crystal-growing vessel  11  and be carried to filter means  17 , in this embodiment, the filter means consist of a pressure vessel  17  provided with mixing means  17   b  and a filter  17   a , which filter  17   a  forms a screen in the pressure vessel  17  for a discharge conduit  18 . 
     Said filter means  17  extract part of the mother liquor solution from the crystal suspension mixture. Part of the concentrated mother liquor solution in the circulated crystal suspension mixture is extracted via the filter. Said extraction leads to a slight increase of the crystal fraction in the crystal suspension mixture in the conduit portion  16   c . In the closed system according to the invention, the extracted mother liquor solution is immediately substituted for new solution from the supply vessel  1 : the net effect on the crystal fraction in the crystal-growing vessel is zero, therefore. Extracted mother liquor solution is carried to the supply means  23  of the crystal-growing vessel  21  of the second concentration stage  20  via the conduit  16 . 
     The advantages of such a construction include a cheaper crystal-growing vessel functioning without pressure, which, owing to the absence of filters, is easy to clean by means of spray nozzles in a manner which is common in the foodstuffs industry. In addition to that, the bypass conduit, which is according to the invention provided with filter means can easily be isolated from the bulk of the system volume for maintenance and cleaning purposes, and the filter means are loaded to a much smaller extent (minimally by a factor of 10) than the crystallization apparatus according to EP- 0 051 340, as a result of which the filter means are much less liable to become clogged by crystals an non-dissolved particles. This makes it possible to operate the apparatus, and in particular the crystal-growing vessel, in such a manner that a high ice crystal fraction can be achieved and the capacity of the separating means increases. 
     The extracted mother liquor solution is introduced into the crystal-growing vessel  21  as the supply solution  23  via the conduit  18 , which crystal-growing vessel  21  is likewise provided with a conduit  22  and a circulation pump  22   a  incorporated therein, as well as with a heat exchanger  22   b  of the scraped surface type. Thus, crystals are formed in the mother liquor solution again, which crystals can grow further in the crystal-growing vessel  21  and be returned to the crystal-growing vessel  11 , via the intermediate pipe  30 , by means of the pump element  30   a.    
     Thus, a highly concentrated mother liquor solution is formed in the crystal-growing vessel  21 , whilst a low-concentration mother liquor solution is formed in the crystal-growing vessel  11  of the first concentration stage  10 . 
     Analogously to the first concentration stage  10 , also the crystal-growing vessel  21  of the second concentration stage  20  is provided with a circulation conduit  26 , in which a circulation pump  26   a  is incorporated. Filter means  27  are arranged in this circulation conduit  26 , which filter means are likewise provided with mixing means  27   b  and a filter  27   a . Also said filter means  27  extract mother liquor solution from the crystal suspension mixture formed in the crystal-growing vessel  21 , which higher-concentration mother liquor solution is discharged via the conduit  31  as a final product (possibly as a starting product for a third concentration stage (not shown)). The crystals that are present in conduit portion  26   c  are returned to the crystal-growing vessel  21  again. 
     In order to prevent clogging of the filters  17   a  and  27   a , filter cleaning means may be present in the pressure vessels  17  and  27 , respectively, which filter cleaning means preferably consist of a scraping element which is movable with respect to the filter surface. 
     The apparatus that is shown in FIG. 2 shows a preferred embodiment in which Identical parts are indicated by the same reference numerals. In this preferred embodiment, the separating means  17  are incorporated in the washing column  15 , in which the conduit  14   b  that now corresponds to the conduit  18  according to FIG. 1 not only returns to the first crystal-growing vessel  11  of the first concentration stage but also, via a three-way valve  40 , to the crystal-growing vessel  21  of the second concentration stage  20  in the form of the supply conduit  14   c , thus effecting the transport of crystal suspension mixture from the crystal-growing vessel  21  to the crystal-growing vessel  11 , as a result of which the pump element  30   a  of FIG. 1 is no longer required. 
     Also the crystal-growing vessel  21  is provided with a circulation conduit  26  analogously to the embodiment of FIG.  1 . 
     FIG. 3 shows an embodiment of a separating apparatus or washing column apparatus for separating crystals from a crystal suspension mixture. The washing column apparatus consists of a cylinder  50  having a closed side  50   a . Present inside the cylinder  50  is a piston  51 , which can be reciprocated in the cylinder by means of a piston rod  52  and driving means (not shown). The piston  51  shuts off a washing column chamber  54  during said movement. The piston  51  is also provided with filter means  51   a.    
     When the piston  51  makes a suction stroke, a particular amount of crystal suspension mixture is introduced into the washing column chamber  54  from the crystal-growing vessel  11  (see FIGS. 1 and 2) via the supply line  63  that corresponds to the reference numeral  14   a  in FIGS. 1 and 2 and via the hollow piston rod  52 . When the piston makes a compression stroke or delivery stroke, the crystal suspension mixture that has been introduced into the chamber  54  will be compressed by the piston, as a result of which the crystals that are present will be compacted against the side  50   a  of the cylinder. The mother liquor solution can leave the chamber  54  via the filter means  51   a  through the piston  51  and be returned to the crystal-growing portion of the crystallization apparatus, for example to the crystal-growing vessel  11  of FIG. 1, via the discharge conduit  60 , which corresponds to the conduit  14   b  in FIGS. 1 and 2. 
     The crystal bed, for example ice crystals, that has been compacted in the washing column chamber  54  is introduced from the washing column chamber  54 , via connection  56   a , into a circulation conduit  56  of a so-called washing circuit by removing means  55  preferably consisting of a rotary disc  55   a  comprising scraping knives, A heating element  57  is incorporated in said circulation conduit  56 , which heating element causes the removed crystals to melt in whole or in part, and the obtained liquid is reintroduced into the washing column chamber  54 , via connection  56   b , by means of a pump  58 . The valve  59   a  in the discharge conduit  59  is closed against entry of the purified, removed crystals. 
     The washing liquid thus obtained, which was initially removed as crystals by the removing means  55 , now serves to wash out the crystal bed, which is still in the compacted condition effected by the piston  51 . The washing liquor displaces the mother liquor solution that is present between the crystals in the direction of the piston  51 . Thus, a separation interface or wash front is formed in the compacted crystal bed between the part of the crystal bed near the removing means  55 , in which the washing liquor has displaced the mother liquor solution that is present between the crystals, and the part of the crystal bed near the piston  51 , in which the mother liquor solution is still present between the crystals. The mother liquor solution can be discharged from the washing column chamber  54  via the filter means  51   a  through the piston and from the washing column apparatus via the conduit  60 . 
     While the mother solution is being displaced by means of the washing liquid, the wash front moves in the direction of the piston  51 . The location of the moving wash front will be detected by the sensor  65  at some point, which is the signal for the opening of the valve  59   a  and the removal of the pure, washed portion of the crystal bed above the wash front from the washing column chamber  54  by means of the piston  51  and the washing circuit  56 ,  57 . 
     Although the current apparatuses function indeed, the compacted crystal bed may encounter such a resistance with the cylinder walls that the pressure exerted on said compacted crystal bed by the piston rod and the piston is insufficient for moving said bed in the direction of the removing means  55 , so that the separation and purification of the compacted crystal bed by the washing liquid obtained in the washing circuit  56  is inadequate. 
     As a result of said friction, the piston  51  is unable to build up sufficient washing pressure, because the compacted bed of solid matter, for example organic crystals or ice crystals, is wedged within the cylinder. To this end, a pressure means in the form of a restriction valve is incorporated in the circulation conduit  56  of the washing circuit in FIG. 3 b , which is a larger-scale view of an embodiment of a washing column according to the invention. Thus an additional differential pressure can be built up from the washing circuit by means of said restriction valve  61  and a circulation pump  58 , which makes it possible for the compacted crystal bed to be washed by means of washing liquid. 
     Since the washing column apparatus can be operated over a wider range if a higher washing pressure is used, its capacity and thus the yield of solid matter in the form of organic crystals or ice crystals will increase. The pressure means  61  may be in the form of an expansion vessel  65  incorporated in the conduit  56 , as is shown in FIG. 3 c . The expansion vessel  65  is separated by a membrane  65   a , and an air pressure can be created in the space  66  by means of a compressor  67 , which pressure can be exerted as a differential pressure on the compacted bed in the washing column chamber  54  via the membrane  65   a  and the washing liquid that is present in the conduit  56 . 
     The differential pressure may also be generated by means of an external pump. 
     Preferably, the pressure means (see FIGS. 3 b  and  3   c ) can be controlled by means of a sensor  62 ,  62   a , which detects the position of the wash front in the compacted crystal bed on the basis of, for example, a temperature difference, or an the basis of an optical signal or an electric conductivity value. Thus, adjustment of the pressure means  61  can take place on the basis of the position of the wash front, so that a maximum and optimally efficient crystal yield is obtained with the differential pressure imposed from the washing circuit  56 . 
     FIG. 4 furthermore shows another embodiment of a washing column apparatus according to the invention. In this embodiment, which need not necessarily be combined with the washing column apparatuses as shown in FIGS. 3 a - 3   c , a plate member  51   b  which moves along with the piston  51  is fitted to the piston rod  52 , which plate member  51   b  divides the space defined by the piston  51  and the piston rod side  50   b  of the cylinder  50  into a first filtrate chamber I defined by the piston  51  and the plate member  51   b , and a second filtrate chamber II defined by the plate member  51   b  and the piston rod side  50   b  of the cylinder  50 . Furthermore, the discharge conduit  60  for filtered liquid extending from the second filtrate chamber II includes a conduit  60   a  leading to the first filtrate chamber. 
     Valves  71  and  70  are arranged in, respectively, the conduit  60   a  and the discharge conduit  60 , which valves are capable of shutting off the conduits. The valve  70  is preferably a three-way valve (indicated by reference numeral  40  in FIG.  2 ), which is capable of switching between the conduit  60   b , which leads to the crystal-growing vessel  11  of the first concentration stage  10  as the conduit  14   d  in FIG. 2, and the conduit  60   c , which leads to the crystal-growing vessel  21  of the second concentration stage  20  as the conduit  14   c  in FIG.  2 . 
     This specific washing column apparatus comprises two filtrate chambers I and II which are separated from each other by the dividing plate  51   b , which dividing plate preferably seals the two chambers against the cylinder wall in a non-critical manner. During a compression stroke (the upward movement of the piston rod  52 , the dividing plate  51   b  and the piston  51 ), liquid is supplied from the washing column chamber  54 , through the filter means  51   a  and the piston  51 , via the first filtrate chamber I, the conduit  60   a  (with the valve  71  in the open position) and the conduit  60  to the expanding second filtrate chamber II. During this compression stroke, the two chambers I and II are in open communication with the crystal-growing vessel  10  of the crystallization apparatus of FIG. 2 via the open three-way valve  70  and the discharge conduit  60   b.    
     During the suction stroke (downward movement of the piston rod  62 , the dividing plate  51   b , and the piston  51 ), the three-way valve  70 , and thus the combined discharge of the conduits  60  and  60   a , is fully closed for a short period of time. As a result, liquid is forced back from the second chamber II to the first chamber I via the conduit  60   a  and through the filter  51   a . This pressure surge ensures that the compacted crystal bed is detached from the filter  51   a  and does not disintegrate during the suction stroke. After said short period of time, the valve  71  is closed and the three-way valve  70  is switched in such a manner that the mother liquor solution from the second filtrate chamber II is returned for crystallization either to the crystal-growing vessel  11  of the first stage  10 , via the conduit  60   b  ( 14   d ), or to the crystal-growing vessel  21  of the first stage  20  of the crystallization apparatus according to FIG. 2, via the conduit  60   c  ( 14   c ). 
     In the latter case, an equal volume of crystal suspension is forced from the second crystal-growing vessel  21  to the first crystal-growing vessel  11  via the conduit  30  (see FIG.  2 ). Said latter vessel can expand in the direction of the supply vessel  1  via the conduit  13   a  in that case. In this way, the desired transport of crystals from the second crystal-growing vessel  21  (high concentration) to the first crystal-growing vessel  11  can be controlled by means of the three-way valve  70 .