Abstract:
A difference in pressure is generated between the two sides of a filter medium ( 1 ) so that the liquid ( 2 ) charged with biopolymers ( 3 ), for instance xanthan, that is fed through the inlet ( 5 ) goes through the filter medium ( 1 ), wherein the biopolymers ( 3 ) are retained by the filter medium ( 1 ). As opposed to crossflow filtration, the main direction of movement ( 4 ) of the liquid ( 2 ) is determined by the difference in transmembrane pressure. A cathode ( 7 ) is arranged under the filter membrane ( 1 ). A membrane serving as anode ( 8 ) is arranged on the opposite side of the process chamber ( 9 ). An electrical field is built up between the electrodes ( 7, 8 ). Due to the fact that the biopolymer components ( 3 ) carry a negative surface charge, a force moving in the direction of the anode ( 8 ), and, hence, against the main direction of movement ( 4 ) of the liquid ( 2 ), impinges upon said components in the electrical field ( 8 ), whereby the concentration of biopolymers is reduced in the surroundings of the filter medium ( 1 ) and filtration speed is increased. A surprising, additional effect is that the electrical field leads to reinforced coagulation tendency of the biopolymers ( 3 ) which further favors filtration by the formation of agglomerates.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a method for the separation of biopolymers from a liquid, the procedure comprising a filtration step. The invention further relates to a filter apparatus for the separation of a disperse phase, and in particular of biopolymers, from a liquid.  
         BACKGROUND OF THE INVENTION  
         [0002]    Among the most industrially important biopolymers is the polysaccharide xanthan. A common method for the separation of xanthan is described by Y.-M. Lo, S.-T. Yang and D. B. Min in “Ultrafiltration of Xanthan Gum Fermentation Broth: Process and Economic Analysis” (Journal of Food Engineering, 32, 219-237 (1997)). A xanthan containing biosuspension is concentrated by means of an ultrafiltration unit, and treated with isopropanol in order to precipitate the xanthan. Typically, the ultrafiltration is a cross flow filtration, that is the main direction of motion of the suspension liquid occurs perpendicular to the direction of filtration, thus essentially parallel to the filtration medium. This is necessary to prevent the filtration medium from blockage and clogging, which is commonly known as “membrane fouling”. Membrane fouling greatly reduces the permeability of the filtration medium, and possibly almost completely disrupts the filtration process. The major part of the operating costs for the ultrafiltration is caused by the energy consumed by the pumps providing for motion of the liquid. Since the xanthan content in the liquid increases during the process of ultrafiltration, the viscosity of the concentrated suspension increases as well, causing an increase in the necessary pumping capacity. Ultrafiltration merely achieves an increase in xanthan concentration, but does not provide for actual separation of xanthan, since it is necessary that the xanthan containing suspension liquid remains well pumpable, as to maintain the functioning of the cross flow filtration apparatus.  
           [0003]    Subsequent addition of isopropanol to the concentrated suspension causes precipitation of xanthan, which then is collected by means of filtration or centrifugation. After the step of filtration or centrifugation, the isopropanol is recovered by distillation. The process of distillation is fairly energy consuming, since it is necessary to provide heat of vaporization for the whole amount of alcohol used during precipitation. On the other hand, it is not possible to forego the step of distillation, in order to recycle a major part of the isopropanol, and to avoid high alcohol concentration in wastewater and high consumption of isopropanol. However, loss of the organic solvent isopropanol is inevitable when following conventional procedures, since alcohol is also contained in the xanthan fraction, separated by filtration or centrifugation, respectively.  
         DESCRIPTION OF THE INVENTION  
         [0004]    Due to the problems related to recycling of alcohol as well as loss of alcohol, it is an object of the present invention to provide a method for separation of biopolymers from a liquid, and especially for the separation of xanthan, which does not necessitate a step of precipitation with isopropanol, and which uses a significantly lower amount of total energy, compared to common procedures.  
           [0005]    This goal is achieved by providing a method for separation of biopolymers form a liquid, and especially for separation of polysaccharides such as xanthan or for separation of poly hydroxybutyric acid, which contains a step of electric field pressure filtration. During the step of electric field pressure filtration, a pressure differential is built up between both sides of the filtration medium, the filtration medium being suitable for filtration of biopolymers. Additionally, an electric filed is applied in the surrounding of the filtration medium in such a way that a force acting on the biopolymers is created, which operates in a direction opposite to the main direction of motion of the liquid containing the biopolymers. Therefore, the main direction of motion of the liquid within a filtration cavity is fixed in a direction extending through the filtration medium, and not across the filtration medium, as it is the case for cross flow filtration. The biopolymers, which carry a charge due to dissociated functional groups, experience a force away from the filtration medium caused by the applied electric filed. The electric field is oriented in way that lines of electric flux run in a direction perpendicular to a surface of the filtration medium, or form an acute angle with a surface of the filtration medium. Electrokinetic effects occur, and biopolymers are moved away from the filtration medium by a process of electrophoresis. This causes a lowering of biopolymer concentration in the vicinity of the filtration medium. Further, the kinetics of filtration is increased due to reduced viscosity and prevention of pore blocking within the filtration medium. The liquid used is mainly an aqueous medium; the use of organic solvents is not needed.  
           [0006]    An additional and surprising effect is the enhanced tendency towards coagulation exhibited by the biopolymers, caused by the electric field. This in turn favors the filtration through formation of agglomerates.  
           [0007]    Further surprisingly, in the production of xanthan according to a method of the present invention, it is possible not only to forego the use of an organic liquid and thus to avoid the related step of distillation, but also to forego the preceding step of ultrafiltration using a cross flow filtration apparatus. This is beneficial for the operating costs as well as for the investment cost for a xanthan production facility, since the use of a cross flow filtration apparatus is avoided.  
           [0008]    Preferably a membrane is used as the filtration medium, which advantageously is an ion-exchanging membrane. Alternatively, a filtration fabric or tissue, or a rigid porous compound is used as filtration medium.  
           [0009]    In an especially advantageous embodiment of the present invention, the pressure differential is larger than the difference between atmospheric pressure and vacuum. This causes an increase in filtration speed.  
           [0010]    The pressure differential is advantageously created by hydrostatic pressure exhibited by a fluid, in a hydraulic fashion by at least one pump, by means of gaseous pressure differential, or by the radially hydrostatic pressure built up due to centrifugal forces.  
           [0011]    In an especially advantageous embodiment of the present invention, the step of filtration is performed with an apparatus, in which one or more hollow support elements are disposed within a chamber, and equipped with a filtration medium. The chamber contains an inlet through which is introduced a xanthan-charged liquid. The pressure differential is built up between the exterior and the interior of the support element, and the main direction of movement of the liquid is therefore defined to occur from the exterior to the interior and through the filtration medium. The liquid runs off the interior of the support element through a filtrate drain. By connecting at least two electrodes with an electric voltage source, an electric field is created. The electrodes are arranged in a way that a force is created in the vicinity of the filtration medium, acting on the biopolymers and operating in a direction opposite to the main direction of motion of the liquid.  
           [0012]    According to another advantageous embodiment of the present invention, the step of filtration is performed in a filter press equipped with at least two electrodes, or in a pressure filtration apparatus equipped with at least two electrodes. When working with small batches, the step of filtration is preferably performed within a suction filter equipped with at least two electrodes.  
           [0013]    According to another advantageous embodiment of the present invention, the method contains a step in which the ion concentration of the liquid is lowered. This step is preferably performed before the step of filtration, and is advantageously achieved using an ion exchanger, or by dialysis or electrodialysis. Advantageously, the pressure differential and the electric field are applied at the same time.  
           [0014]    The method according to the present invention is especially advantageous in cases when the liquid contains additional solid particles besides biopolymers. According to the instant invention, the additional solid particles are advantageously separated from the liquid either during the step of filtration or before the step of filtration, preferably by centrifugation. In case the additional solid particles are separated from the liquid during the step of filtration, the presence of the electric fields is beneficial for the separation of the solid particles. Since solid particles in an aqueous medium in general carry a surface charge, electrophoretic effects are at work which defer build up of filter cake of solid particles on the filtration medium. This causes a beneficial effect for the kinetics of the filtration process.  
           [0015]    Preferably, the filtration medium is disposed between at least one pair of electrodes, a pair consisting of anode and cathode. Advantageously, the anode is at least partially made of a nickel based alloy, graphite or platinum. One of the electrodes possibly is a metallic support beneath the filtration medium.  
           [0016]    In another advantageous embodiment of the instant invention, the filtration medium is at least partially formed from an electrically conducting material and is itself used as an electrode.  
           [0017]    Providing a filtration apparatus for separating a disperse phase, and especially biopolymers, from a liquid, provides a further solution to the underlying problem that makes the present invention useful. The filtration apparatus comprises one or more hollow support elements equipped with a filtration medium, the hollow support elements arranged within a chamber for receiving a liquid charged with a disperse phase through an inlet within the chamber. Between the exterior and interior of each support element a pressure differential can be created, which defines the main direction of motion of the liquid from the exterior of the support element to its interior. Furthermore, the filtration apparatus comprises at least two electrodes, which are arranged in such a way that by connecting the electrodes with an electric voltage source an electric filed can be applied, which causes a force acting on the disperse phase in a direction opposite of the direction of main motion of the liquid. The liquid runs off from the interior of the support element through a filtrate drain.  
           [0018]    In a preferred embodiment of the present invention, a support element is provided in the shape of a cylinder or a prism, and the filtration medium at least partially covers the generated surface of said support element. Advantageously, an electrode is annularly disposed around the support element.  
           [0019]    In another preferred embodiment, each support element is provided in the shape of a plate, disc or convex disc, having two abutting faces, wherein the filtration medium covers at least partially at least one of the two abutting faces. Advantageously, the support elements are displaced either horizontally or vertically.  
           [0020]    In yet another preferred embodiment, at least one electrode is integrated into each support element. Preferably the at least one electrode is provided in the shape of a plate, disc or convex disc.  
           [0021]    Preferably, the filtration apparatus comprises a plurality of support elements arranged for operating in parallel fashion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    The present invention is now described in conjunction with the following drawings, in which exemplary embodiments are displayed. The drawings are schematized for the sake of clarity, and are not according to scale.  
         [0023]    [0023]FIG. 1 displays a schematic diagram illustrating the basic principles governing the step of filtration according to the instant invention;  
         [0024]    [0024]FIG. 2 displays a schematic side cross section of a filtration apparatus according to the present invention, not shown to scale, the filtration apparatus having convex-disc-shaped support elements; and  
         [0025]    [0025]FIG. 3 displays a schematic side cross section of a filtration apparatus according to the instant invention, not shown to scale, the filtration apparatus having a cylindrical support element. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    In FIG. 1, the basis principles of a method according to the instant invention are schematically illustrated. A pressure differential is created between two sides of a filtration medium  1 , for example a membrane suitable for the filtration of biopolymers. An aqueous liquid  2  charged with macromolecules or colloids formed from a biopolymer to be separated  3 , for example xanthan, enters through inlet  5 . The pressure differential causes the aqueous liquid  2  to penetrate through the filtration medium  1 , and therefore initiates a process of filtration, during which the biopolymer  3  is prevented from passing through the filtration medium  1 . The main direction of motion  4  of the liquid  2  is defined by the pressure differential across the membrane, contrary to a cross flow filtration process. The pressure differential is, for example, applied through inlet  5  and in a hydraulic fashion using a pump  6 . Underneath the filtration medium  1  there is disposed a metallic support  7  operating as cathode. On the opposite side of the filtration cavity  9 , there is disposed a plate operating as anode  8 , and, for example, is manufactured from Hastelloy. The cathode  7  is connected with the negative pole  10  of a source for direct current  12 , and the anode  8  is connective to this positive pole. This way, an electric field is generated between the two electrodes. Since parts of the biopolymers  3  carry a negative surface charge due to dissociated OH-groups, a force in direction towards anode  8  and opposite the main direction of motion of the liquid is  2  acting on the biopolymers. Above a critical electric field strength, which is required so that the electric field force overcomes a resistance force exhibited by the liquid  2  running off through the filtration medium  1 , the parts of the biopolymers  3  move in an electrophoretic fashion towards the anode  8 . This process causes a lowering of the concentration of biopolymer in vicinity of the filtration medium, and enhances speed of filtration. In addition, due to the law of conservation of electroneutrality of a given system, the liquid  2  is positively charged, and also experiences an electric force. This electric force causes a process of electro-osmosis and supports the movement of the liquid in direction of the cathode  7 , superimposing an electro-osmotic pressure onto the hydraulic pressure differential. After completion of the separation the biopolymer mass remaining in the filtration cavity  9  is possibly dewatered by application of a gaseous pressure differential, and is subsequently collected.  
         [0027]    In order to maintain a low flow of an electric current, the conductivity of liquid  2  is reduced using an ion exchanger (not shown), before subjected to the above-described filtration process.  
         [0028]    In FIG. 2 displayed is a schematic cross sectional view, not to scale, of a filtration apparatus according to the instant invention. The hollow, convex-disc-shaped support elements  101 ,  102 ,  103  are made from electrically non-conducting plastic material, and are disposed inside of chamber  104 . The support elements  101 ,  102 ,  103  are affixed to hollow shaft  105 . The interior of each of the support elements  101 ,  102 ,  103  is connected with the interior of hollow shaft  105  through filtrate draining bores  106 ,  107 ,  108 . Each of the support elements  101 ,  102 ,  103  displays on its exterior surface, the exterior surfaces having openings  109 ,  110 ,  111  extending through the surfaces, a filtration medium  112 ,  113 ,  114 , which is for example a membrane suitable for filtration of biopolymers. By applying a vacuum at the filtrate side, that is applying a vacuum to a cavity in which filtrate is collected, and which is connected to the interior of the hollow shaft, a pressure differential is created between the interior and exterior of each of the support elements  101 ,  102 ,  103 . Inside each of the support elements  101 ,  102 ,  103  and below the filtration medium there are disposed electrodes  115 ,  116 ,  117 , which are connected via a shielded cable  118  disposed within an isolating body  125 , and a slip ring  119  with the negative pole  120  of a direct current source  121 . Therefore, the electrodes are connected to operate as cathodes  115 ,  116 ,  117 . Opposite each cathode, and on the other side of the corresponding filtration medium  112 ,  113 ,  114 , there are disposed electrodes  122 ,  123 ,  124 . The electrodes are connected via a shielded cable  126  disposed within an isolating body  125 , and a slip ring  128  with the positive pole  127  of a direct current source  121 , and act therefore as anodes  122 ,  123 ,  124 . Therefore, an electric field is applicable between the pairs of electrodes  115 / 122 ,  116 / 123 , and  117 / 124 . The hollow shaft  105  is supported by cantilever bearings. Only one bearing  130  is explicitly shown, which is sealed against the filtration cavity  132  by way of labyrinth-sealing. The chamber can be opened through flanged connection  133 . In addition, the chamber comprises an inlet  134  with inlet flange  135 , and an outlet  136  with outlet flange  129 .  
         [0029]    In operation, a liquid charged with a disperse phase such as xanthan is introduced into the filtration cavity  132  of chamber  194  through inlet  134 . The liquid is filtered through filtration medium  112 ,  113 ,  114  by means of an applied pressure differential, enters the interior of support elements  101 ,  102 ,  103 , and is guided through filtrate draining bores  106 ,  107 ,  108  and through the interior of hollow shaft  105  to a filtrate collecting container (not shown). Outlet  136  is closed. A force operates in a direction opposite to the main direction of motion of the liquid on the disperse phase, which lowers the concentration of the disperse phase in vicinity of filtration medium  112 ,  113 ,  114 , and enhances the speed of the filtration process. The disperse phase is precipitated at anodes  122 ,  123 ,  124  and together with the last portion of liquid running off the filtration cavity  132  also at filtration medium  112 ,  113 ,  114 . After completion of the filtration, the precipitated disperse phase is collected, and to this end, a torsional vibration is applied to hollow shaft  105  and support elements  101 ,  102 ,  103 . The precipitated disperse phase spins off the support elements  101 ,  102 ,  103 , and slides down slanted wall  137  into the lower part of chamber  104 , and towards outlet  136 .  
         [0030]    In FIG. 3 displayed is a schematic cross sectional view, not to scale, of another filtration apparatus according to the present invention.  
         [0031]    The apparatus contains a hollow cylindrical support element  202 , which has openings  201 , and which is covered with a filtration medium  203 , such as a membrane suitable for filtration of biopolymers. The support element is disposed inside a pressure container  204 . Along the container wall  205  there is disposed a cylindrical electrode  206 , electrically isolated from the container wall  205  through an isolating layer  207 , and surrounding the support element. Electrode  206  is connected with direct current source  211  through an electric cable  208  that is guided via isolating body  209  through the wall of pressure container  204 . The electrode  206  is connected with the positive pole  210  of direct current source  211 , and operates therefore as anode  206 . Inside of the support element  202  there is disposed a rod-shaped electrode  212 , supported by two isolating pieces  213  and  214 , the isolating pieces disposed within support element  201  and lid  215 , respectively. The rod-shaped electrode  212  is connected via electric cable  216  with negative pole  217  of direct current source  211 , and therefore operates as cathode  212 . An inlet  218  provides access to filtration cavity  219 . The inlet  218  extends through lid  215 . Lid  215  can be separated from the remaining part of pressure container  204  by means of flange  221 . An outlet  220  provides access from the interior of support element  202  to a filtrate draining pipe (not shown).  
         [0032]    In operation, a liquid charged with a disperse phase such as xanthan is pumped through inlet  218  into the interior of the pressure container  204 . The pump (not shown) also creates a pressure differential between interior and exterior of support element  202 , being the driving force for the main direction of motion of the liquid extending through the filtration medium  203 . The liquid enters the interior of support element  202  through openings  201 , and the filtrate exits the interior through outlet  220 . The electric field created between electrodes  206  and  212  exhibits a force on the disperse phase in a direction opposite the main direction of motion of the liquid. Therefore, the concentration of the disperse phase in the vicinity of the filtration medium  203  is lowered, causing an increase in filtration speed, and preventing clogging of pores of the filtration medium  203 .