Abstract:
The invention relates to an improved separation device in which a plate unit containing the filter comprises a separate abutment ring and a cassette which can be easily connected thereto, said cassette comprising at least one filter together with, if required, a frame belonging thereto so as to support the filter. If the filter consists of a flexible membrane, this is fitted into a special frame. However, if the filter is a rigid self-supporting filter of a porous material, for example, a ceramic filter or the like, a separate frame is not needed to support the filter.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device based on cross flow membrane filtration for separation of separable constituents from a liquid medium. Additionally the invention relates to a filter cassette and an abutment ring intended to be used in said separation device. 
     In a cross flow filter a liquid is supplied to a chamber comprising at least one dynamic membrane. The liquid being supplied is divided into a permeate flow penetrating the dynamic membrane and a reject flow flowing along the filter without penetrating it. 
     BACKGROUND OF THE INVENTION 
     Cross flow filtration apparatuses are described in the patent literature, for example, in the Swedish Publication Prints SE 451429, 457607, 459475, 463241 and 465040. Methods based on the use of such apparatuses are successfully applied for separating solid constituents from a liquid phase and for separation of liquid from liquid, for example, for separation of oil from water. The above mentioned Publication Print SE 463241 describes such a fluid/fluid separation method. 
     SE 459475 describes a separation device comprising several adjacently arranged cells. FIG. 1 shows a section of such a stack. Each unit or cell consists of a plate  10  provided with a central opening through which extends a shaft  11  provided with rotor blades  12 . On each side of the plate  10  a filter  15  is fastened by means of clamp rings  13  and  14 . The liquid to be treated is supplied through a common channel  16  formed by through-holes in the plates  10 . The liquid stream is supplied into the chamber  17 , whereby part of it (the permeate) passes through the filter and is gathered via channels  18  in the plate to be finally discharged through a common outlet (not shown). The portion of the liquid flow (the reject) which does not penetrate through the filter, is discharged through holes in the plate. Said holes form a common conduit  19  for the outgoing reject. The liquid is pushed across the filter surface  15  by means of the rotor blades  12 . 
     When the filter is changed in the above mentioned device the plate containing the filter to be changed is taken out. The removing of the plate itself from the stack is carried out relatively easy in a way more closely described in the Publication Print in question. However, the removal of the used filter from the plate and inserting of a new filter is very time consuming and hard because the filter is fastened by means of numerous screws to the clamp rings securing the filter to the plate. This causes for its part long shutdowns in the separation plant. 
     SUMMARY OF THE INVENTION 
     According to the present invention a separation device is achieved in which it is possible to change the filters in a very easy and fast way. The invention is based on the idea of instead of fastening the filter directly onto the plate it is fastened on a frame constituting a separate unit or a so called filter cassette. Alternatively, the filter can be a rigid, self-supporting filter of a porous material, for example, a ceramic filter or the like. In this case a separate frame is not required in order to support the filter, instead the filter itself comprises the cassette. The cassette can easily be fitted in an abutment ring intended for the purpose. Thus the abutment ring and the cassette replace the above described plate provided with filters. The filter cassettes can be produced in advance and kept in stock to be at hand when the filter needs to be changed. 
     An essential advantage of the present invention is that the abutment rings together with the end plates, between which the abutment rings are pressed together, form the outer cover of the apparatus. Said construction is very advantageous with respect to mounting, especially when it comes to high filtration units. If the filtration unit would be positioned inside a separate cover the mounting of the cover would require a free space above the filtration unit corresponding at least to the height of the cover. Alternatively it could be possible to divide the cover vertically in separate portions, but such a solution would not be appropriate in practice. The abutment ring according to the present invention forms a portion of the outer cover of the apparatus corresponding to the thickness of the ring. The fitting and removal of said abutment rings requires minimal working space. Another essential feature of the abutment rings is that they form channels for the liquid streams in the separation device. Adjacent abutment rings form a common channel for the ingoing liquid flow, another common channel for the outgoing reject flow and a third common channel for the outgoing permeate. In addition each separate abutment ring contains channels for distributing incoming liquid over the filter and for gathering the reject flow which has not passed through the filter. Additionally each abutment ring comprises a channel for gathering permeate from the filter. A third essential feature of the abutment rings is that they form the frame for the separate cassettes, whereby the thickness of the abutment rings determines the mutual distance between the separate cassettes. 
     The invention will be described in greater detail with reference to the enclosed drawings, in which 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a section of a separation device according to prior art, 
     FIG. 2 shows a section of a separation device according to the invention, 
     FIG. 3 shows an enlargement of a detail in FIG. 2, 
     FIG. 4 shows a partial section of a filter cassette, 
     FIG. 5 shows a cross-section of the filter cassette along the line A—A in FIG. 4, 
     FIG. 6 shows an abutment ring for the filter cassette, 
     FIG. 7 shows a cross-section along the line B—B in FIG. 6, 
     FIG. 8 shows a cross-section of the abutment ring along the line C—C in FIG. 6, 
     FIG. 9 shows an abutment ring according to FIG. 6 together with a filter cassette according to FIG. 4 fitted into it, 
     FIG. 10 shows an alternative embodiment of the support plate for the filter cassette, 
     FIG. 11 shows a cross-section of the support plate according to FIG. 10, and 
     FIGS. 12A-12C showing sections of the separation device illustrate alternative ways of arranging the liquid flow in said device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 shows a section of a separation device according to one embodiment of the invention. The separation device consists of a number of adjacently placed cells or units. The stack can comprise horizontal units, as shown in the figure, or vertical units, which is advantageous in liquid/liquid separation. The units comprise an abutment ring  20  to which a filter cassette  30  is connected, whereby the cassette at its periphery is fitted between two adjacent abutment rings. The cassette, which is shown more closely in FIGS. 4-5, is a circular plate with a center opening  31  for the rotor shaft  11  supporting rotor wings  12 . Clamp rings  32  extend around the center opening  31  and act as a sealing and keep the cassette together. In the embodiment shown in the figure the cassette contains two filters  15 . The liquid to be treated is supplied (not shown in FIG. 2) into the chamber  17 , i.e. the area which according to this embodiment is delimited in the direction of the stack by the filter surfaces between the two cassettes. (The chamber is delimited by the filter of the cassette and the end plate at the ends of the stack). The permeate (P) is conveyed out to the periphery of the cassette and flows into the permeate channel  21  of the abutment ring, said channel extending along said abutment ring. The permeate is discharged through a common channel  23  formed by through-holes in the adjacent abutment rings  20 . The reject is discharged from the chamber  17  through a common channel which is also formed by through-holes in the adjacent abutment rings (not shown in FIG.  2 ). The periphery of the cassette is provided with a sealing  33  (see FIG. 3) so that the liquid chamber  17  is separated from the permeate channel  21 . A sealing groove  22  with a sealing  29  extends in the abutment ring on the outside of the permeate channel (FIG.  3 ). The adjacent units are pressed together between the bottom plate  50  and the cover (not shown) in a manner known per se. 
     FIG. 3 is an enlargement showing more clearly the fitting of the periphery of the cassette between adjacent abutment rings  20 . 
     FIG. 4 shows the filter cassette  30  in a partly sectional view at the periphery. A clamp ring  32  extends around the center opening  31  keeping the inner edge together. Under the filter  15 , which in this embodiment is a flexible membrane, for example, a porous textile cloth, paper cloth or a dynamic membrane of the type described in the above mentioned patent publications, is a support net  40  separating the filter  15  from a massive support plate  41  lying under the support net  40 . Radially directed grooves  42  extend in the periphery of the support plate discharging the permeate from the cassette. A sealing  33  extending along the periphery of the cassette is to some extent directed inwards along each side of the cassette. 
     FIG. 5 shows a cross-section of the cassette  30  at the periphery along the section line A—A in FIG.  4 . In this embodiment the cassette contains two filters  15  separated by support nets  40 , respectively, from the support plate  41  of the cassette. The support net  40  keeps the space between the support plate  41  and the filter  15  stretched so that the permeate can flow towards the periphery where it is gathered in the radial grooves  42  to finally be discharged from the cassette. The end edge  34  of the sealing  33  is provided with radially directed holes  35  for discharging the permeate P from the cassette. The plane sections  36  of the sealing profile are, however, not perforated. The side edges  37  of the sealing profile, which are directed inwards in the direction of the cassette, are curved in against the filter surface in an angle exceeding 90 degrees so that the side edges  37  touch the filter  15  and are obliquely directed outwards towards the periphery of the cassette. By means of this construction the edges  37  will be pressed outwards and obliquely against the filter surface when the sealing profile  33  is pressed together between two adjacent abutment rings. Hereby a tightening of the filter is obtained. In FIG. 5 the liquid flows are indicated with arrows. The incoming liquid flow is designated with I and the outgoing permeate with P. 
     FIG. 6 shows an abutment ring  20  with a through-hole  16  for the incoming liquid flow and another through-hole  19  for the outgoing reject. When several similar abutment rings  20  are placed against each other in a stack, the holes  16  and  19 , respectively, will form through-channels. The holes  16  and  19 , respectively, communicate with the liquid chamber  17  through channels in the abutment ring ( 24  and  25 , respectively). Additionally, the abutment ring has a through-hole  23 , which together with similar holes in adjacent abutment rings forms a common channel for the outgoing permeate. A permeate channel  21  gathering the permeate being discharged from the filter cassette, extends in the same manner as the sealing groove  22  along the whole abutment ring. FIG. 7 shows a section of the abutment ring along the section line B—B in FIG.  6 . FIG. 8, which is a section along the line C—C in FIG. 6 shows also the design of the channel  24  viewed from the side. 
     FIG. 9 shows an abutment ring  20  and a filter cassette  30  fitted into it. The permeate P flowing under the filter  15  is gathered by the permeate channel  21  of the abutment ring to finally be discharged through the outlet  23 . The liquid flow I supplied through the inlet channel  16  and the channel  24  of the abutment ring is spread across the filter surface  15  by means of the rotor wings  12  or similar means. The reject R is gathered from the filter surface  15  and discharged through the channel  25  extending inside the abutment ring  20  and flows into the common reject outlet  19 . 
     FIG. 10 shows an alternative embodiment of the support plate  41  in which said plate comprises two thin plates  41   a  and  41   b  joined together. The sides of the plates  41   a  and  41   b  facing the filter are smooth. The sides of the plates  41   a  and  41   b  facing each other are provided with channels with branches which open into the periphery of the support plate  41 . The liquid which has passed the filter membrane  15  (not shown in FIG. 10) flows into the channels  42  through slits  43  in the plates, said slits communicating with said channels. According to the arrangement in FIG. 10 the channel system is alternately located in the upper plate  41   a  and the lower plate  41   b . This is best seen in FIG. 11 which shows a cross-section of the support plate according to FIG.  10 . 
     It is evident that the channels  42  in plates  41   a  and  41   b , respectively, can be arranged in alternative ways. 
     The flow space between the plates  41   a  and  41   b , respectively, can also be achieved in such a way that a net or a porous material is placed between the plates. 
     Alternatively the compact plates  41   a  and  41   b  can be replaced with a porous support plate which in itself allows a filtrated liquid to flow through. 
     By means of the abutment ring according to the invention it is possible to vary the flow of the liquid to be treated. FIGS. 12A-12C show alternative ways to arrange the liquid flow. FIG. 12A shows parallel driving, where the supplied liquid flows in the common supply channel  16  formed by the abutment rings after which the liquid stream is evenly divided in the channels  24  in the separate abutment rings and flows into the space  17  between the filter cassettes. The reject flow is discharged from the space  17  through channels  25  in the separate abutment rings and flows out in the common outlet channel  19 . 
     A modified version of parallel driving is shown in FIG. 12B where every other inlet channel  24  is blocked with plugs  26  and every other outlet channel  25  is also blocked with plugs  27 , but in such a way, however, that it is either the channel  24  or the channel  25  in an abutment ring which is blocked. In this case the liquid flowing into the space  17  through the channel  24  will flow through the center opening of the filter cassette to the opposite side of the cassette to be then discharged through the channel  25  in the next abutment ring. This embodiment increases the turbulence and thereby improves the filtration. 
     FIG. 12C shows a separation device divided into a number of units connected in series. The division has been achieved by blocking the inlet channel  16  and outlet channel  19 , respectively, by means of a plate  28  between two adjacent units. The center opening has also been blocked by means of a ring  28   a . As a result of this, the reject from the first unit will be the flow supplied into the following unit. By means of this arrangement a high concentration of solid material is achieved in the reject flow which is finally discharged from the separation device. 
     It is obvious that the three ways of driving mentioned above can be combined in different ways in one separation device. 
     The separation device according to the present invention can be used both for separating solid particles, solved high-molecular substances and colloids from liquid and for separating a liquid from another liquid, for example, in the way described in SE 463241. 
     The filter material is chosen according to the separation process to be carried out. The filter membrane itself can be provided with one or more layers of a porous material when required, for example, of the type described in the above mentioned patent publications. 
     In the embodiment described above the filter cassette is fitted between two abutment rings. Naturally the filter cassette can also be fitted in only one abutment ring, but the described alternative is to be preferred. 
       
     The filter cassette does not necessarily have to have a frame comprising a support plate and a support net in the way described above. The components of the frame are chosen on the basis of the properties of the filter membrane, possible porous covering, etc. The essential purpose of the frame is to give the filter stability, ensure that the filter is kept stretched and to enable fastening of the cassette in the abutment ring. Alternatively the filter can be a rigid, self-supporting filter of a porous material, for example, a ceramic filter or the like. In this case a separate frame is not required in order to give the filter stability because the filter itself forms the cassette. 
     In the embodiment according to FIG. 2 the separation device is provided with a rotor. This does by no means mean that the invention is restricted to the employment of a rotor. It is obvious that the rotor can be replaced with another device in order to achieve sufficient liquid turbulence and transport. As examples of such equipment transmitters sending sound waves of varying frequences can be mentioned; transmitters of sound waves of varying frequences together with a device for accomplishing electric fields; and a rotor together with a device for accomplishing electric fields. The above embodiments can also be combined. 
     It is obvious to a person skilled in the art that the various embodiments of the invention may vary within the enclosed claims.