Separation of solid particles of various sizes from viscous liquids
The present invention relates to a process for separating solid particles of various sizes from viscous liquids, for example biomass from liquids of biotechnological processes, by pressure filtration. 
     In the workup of viscous polysaccharide solutions from biotechnological processes, a fundamental problem has to be overcome, namely to separate the biomass from the viscous liquid phase. In the case of the polysaccharide, a B-D-1,3-glucan, formed by fungus of strain ATCC 15205, there are additional problems due to the fact that a not inconsiderable proportion of the product adheres to the mycelium and is structurally similar to cell wall constituents. 
     The raw culture broth has hitherto been worked up by strongly diluting it, then filtering the dilute culture broth through conventional filter material, or centrifuging it, and reconcentrating the filtrate. In this sequence of operations, the dilution step is indispensable since the undiluted culture broth (an inhomogeneous mixture of mycelium, polysaccharide, media constituents and water) very quickly blocks up any conventional filter medium ever investigated or, owing to the high viscosity, renders sedimentation of the biomass through centrifugation impossible. Nor is it possible to separate the culture broth in a two-phase system owing to the structural similarity between the product and the cell wall. 
     Despite dilution, the lifetimes of membranes employed in the filtrations mentioned are only short. Nor does the prior art disclose a more effective process without higher hardware requirements. 
     It is an object of the present invention, starting from the prior art as represented in Ullmann&#39;s Encyclpadie der Technischen Chemie, 4th edition, volume 2, pages 156/157, to separate the biomass from ideally undiluted, raw culture broths. 
     We have fond that this object is achieved according to the invention by subjecting the inhomogeneous or homogenized suspension, without or only after a little dilution, to a pressure filtration whereby the solid particles of various sizes are retained according to size in a plurality of in particular in from 1 to 3, superposed or juxtaposed filter stages on filter layers of various depths and on filter surfaces and are separated from the viscous liquid. 
     Further features of the invention form the subject-matter of the subclaims. 
     In this pressure filtration, the pressure is exerted on the gas phase above the liquid to be filtered or directly on the liquid phase and is in the range from 1 to 10 bar. The filter layers for separating off coarse particles consist of a three-dimensional network of metal wire or synthetic cloth or cellulose material having an average pore width of from 1 to 3 mm and of sufficient depth (from 10 to 100 mm). For example, the density of such a synthetic filter is up to 80 kg/m 3 , while that of the metal version is correspondingly higher. 
     To separate off relatively fine particles the next stage comprises for example an open-pored polyurethane foam (average pore width from 0.2 to 0.8 mm, density up to 80 kg/m 3 ), followed finally for example by silk gauze (mesh size from 0.006 to 0.2 mm) or similar filter surfaces made of natural or synthetic materials. The pore width of the foam can be reduced in size by compressing the foam. 
     This filter technique is a combination of depth and surface filtration. In the upper filter layers first encountered by the flowing medium, the biomass is retained inside the filter, while at the silk gauze and similar filter cloths the pretreated suspension is subjected to a surface filtration. 
     The filter apparatus should be constructed in such a way as to maximize the size of the filter area. The flow on the inside must be influenced by constructional measures in such a way that the suspension cannot escape from passing through the filter layers. 
     This system is flexible and, by proper selection of the filter layers, is adaptable to different problem situations. The system has proved to be of good utility in particular in the case of highly inhomogeneous culture broths of high mycelium content (up to 10 g of dry matter/l) and high polymer concentrations (up to 12 g of dry matter/l) as obtained from shaking flask cultivations. 
     It will be readily understood that the process can also find utility in other fields where viscous liquids are worked up, as for example in the 
     production of celluloid 
     production of cellulose ethers 
     production of cellulosic fibers 
     production of fats and oils 
     production of gelatin 
     production of natural and synthetic resins 
     production of adhesives and sealants 
     production of pectin 
     production of starch 
     production of lubricants 
     production of other polysaccharides (agar, alginate, carrageen, dextran). 
     In what follows, the process according to the invention is further illustrated by reference to a drawing and to various Examples. 
    
    
     The drawing shows a section through a laboratory pressure filter unit used for filtering the various culture broths. 
    
    
     In the pressure filter unit, three different filter layers 1, 2 and 3 are arranged one below the other in the direction of flow or in certain circumstances even side by side. As in existing apparatus these filter layers are arranged on top of a filter support unit 4, into which position they are inserted from below. A coupling ring 5 connects the conical filter outlet 6 to the pressure filter unit and at the same time ties the filter support unit firmly into the pressure filter unit. The culture broth is fed in via line 7 and the filtrate is discharged via line 8. Line 9 is used to supply air or inert gas to apply the necessary pressure on the gas phase above the culture broth or directly on the culture broth. The pressure filter construction described permits rapid and easy replacement of the filter layers; however, this is also true of existing apparatus and not relevant to the invention. Conveniently, two pressure filter units are disposed in a parallel arrangement, so that there is always one unit in operation while at the same time in the other unit the filter layers are cleaned or replaced. It is also technically advantageous to clean the filter stages by back flushing. 
     EXAMPLE 1 
     Three-stage pressure filtration of viscous, inhomogeneous cell suspension 
     A viscous, aqueous cell suspension obtained by culturing of Sclerotium rolfsii ATCC 15 205 has the following composition: 
     Inhomogeneous cell size distribution 0.005 m (hyphae fragments up to 1 cm pellets) in diameter: 
     
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cell dry matter:  4.53 g/l                                                
homoglucan content of                                                     
                  5.12 g/l                                                
aqueous solution:                                                         
viscosity of cell suspension:                                             
                  about 1200 mPas at a shear                              
rate of D = 9s.sup.-1 at 20° C.                                    
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     1 liter of this cell suspension is introduced into the pressure filter unit shown in the drawing and is subjected to a pressure filtration at 20° C. under a positive pressure of 3 bar. 
     Filter structure 
     
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Filter stage 1:                                                           
            Three-dimensional synthetic cloth having                      
            an average pore width of 2 mm and a depth                     
            of 40 mm                                                      
Filter stage 2:                                                           
            Polyurethane foam (density: 60 kg/m.sup.3,                    
            pore width: 0.55 mm, depth: 10 mm)                            
Filter stage 3:                                                           
            Silk gauze (pore width: 0.006 mm, depth:                      
            0.5 mm)                                                       
Filter area:                                                              
            10.7 cm.sup.2                                                 
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     Result 
     The clear filtrate contains no detectable cell mass and has a viscosity at 20° C. and D=9s -1  of 738 mPas and a homoglucan content of 10.0 g/l, corresponding to a homoglucan loss in the filtration of 2.14% by weight. The filtration rate is 5300 l/m 2  h. 
     EXAMPLE 2 
     Single-stage pressure filtration of a viscous, homogeneous cell suspension 
     An inhomogeneous aqueous cell suspension obtained by culturing of Sclerotium rolfsii ATCC 15 205 is mechanically homogenized, the result being a cell size distribution ranging in diameter from 0.002 to 0.005 mm. 
     
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Cell dry matter:  2.38 g/l                                                
Homoglucan content of                                                     
                  2.93 g/l                                                
aqueous solution:                                                         
Viscosity of cell suspension:                                             
                  420 mPas at a shear rate                                
D = 9s.sup.-1 at 20° C.                                            
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     1 liter of this homogeneous cell suspension is introduced into the pressure filter unit shown in the drawing and subjected to a pressure filtration at 20° C. under a positive pressure of 4 bar. 
     Filter structure 
     
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Filter stage 2:                                                           
            Polyurethane foam (density: 60 kg/m.sup.3,                    
            pore width: 0.25 mm, depth: 50 mm)                            
Filter area:                                                              
            10.7 cm.sup.2                                                 
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     Result 
     Slightly opalescent filtrate having a cell particle dry matter content of 0.013% by weight. The filtrate has a viscosity at 20° C. and D=9s -1  of 110 mPas and a homoglucan content of 2.15 g/l, corresponding to a homoglucan filtration loss of 26.6% by weight. The filtration rate is 1703 l/m 2  h. EXAMPLE 3 
     Two-stage pressure filtration of a viscous, homogeneous cell suspension 
     A viscous, aqueous cell suspension obtained by culturing Sclerotium glucanicum CBS 52071 in a vented, mechanically stirred 50 l bioreactor has the following composition: 
     homogeneous cell size distribution of 0.002 mm (mycelium fragment total) in diameter 
     
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cell dry matter:  0.81 g/l                                                
homoglucan content:                                                       
                   4.5 g/l                                                
viscosity of cell suspension:                                             
                  380 mPas at a shear rate                                
D = 9s.sup.-1 at 20° C.                                            
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     1 liter of this cell suspension is introduced into the pressure filter unit shown in the drawing and subjected to a pressure filtration at 60° C. under a positive pressure of 2 bar. 
     Filter structure 
     
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Filter stage 2:                                                           
            Polyurethane foam (density: 60 kg/m.sup.3,                    
            pore width: 0.55 mm, depth: 50 mm)                            
Filter stage 3:                                                           
            Silk gauze (pore width: 0.006 mm, depth:                      
            0.5 mm)                                                       
Filter area:                                                              
            10.7 cm.sup.2                                                 
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     Result 
     The clear filtrate contains no detectable cell mass and has a viscosity at 20° C. and D=9s -1  of 129 mPas and a homoglucan content of 4.23 g/l, corresponding to a homoglucan loss in the filtration of 0.48% by weight. The filtration rate is 8972 l/m 2  h. 
     EXAMPLE 4 
     Two-stage pressure filtration to separate glass milling balls from cell fragments and viscous filtrate 
     60 g of moist cell mass of the fungus strain Sclerotium glucanicum are suspended together with 50 ml of glass milling balls (diameter 0.25 mm) in 140 ml of water and ball-milled to disrupt the cells. The glass milling balls are separated from the suspension of disrupted cells under the following conditions: 
     Filter structure 
     
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Filter stage 2:                                                           
              Polyurethane foam (density: 60 kg/m.sup.3,                  
              pore width: 0.55 mm, depth: 55 mm)                          
Filter stage 3:                                                           
              Silk gauze (pore width: 0.05 mm, depth:                     
              0.5 mm)                                                     
Filter area:  10.7 cm.sup.2                                               
Filtration pressure:                                                      
              1 bar positive pressure                                     
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     Result 
     The glass milling balls are retained quantitatively in the filter system, while the filtrate contains over 96% by weight of the cell detritus. 
     EXAMPLE 5 
     Two-stage pressure filtration for separating an aqueous bacterial suspension 
     2 liters of an aqueous suspension of Streptomyces olivacens having a cell dry matter content of 2.5% is flocculated by crosslinking with glutardialdehyde to give an average floc diameter of from 0.2 to 2.0 mm. From this floc suspension the bacterial flocs are separated quantitatively using the following filter structure: 
     Filter structure 
     
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Filter stage 2:                                                           
              Polyurethane foam (density: 60 kg/m.sup.3,                  
              pore width: 0.55 mm, depth: 55 mm)                          
Filter stage 3:                                                           
              Silk gauze (pore width: 0.1 mm, depth:                      
              0.5 mm)                                                     
Filter area:  10.7 cm.sup.2                                               
Filtration pressure:                                                      
              2 bar positive pressure                                     
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     Result 
     The clear filtrate contains no detectable cell mass.