Filtering unit and method of sealing same

Details Filtering unit and method of sealing same Filtering unit and method of sealing same

2002-04-15

2004-07-06

Kim, John (Department: 1723)

Liquid purification or separation

With repair or assembling means

C210S085000, C210S323100, C210S323200, C210S450000, C210S490000

Reexamination Certificate

active

06758970

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to filtration by means of crossflow technique and filtering elements having a filter membrane on a porous carrier.
Particularly, the invention relates to a crossflow membrane filtering unit having a sealing system including a channel enabling the detection of a leak without the quality of the filtrate being impaired.
BACKGROUND OF THE INVENTION
The removal of solids from a liquid by filtration can take place in several, principally different manners. In so-called dead end filtration, a suspension is driven in its entirety towards a filter, and a filter cake is formed which the filtrate penetrates. In cross-flow filtration, the mixture to be filtered is pumped across the filter clement in a circuit, whereby part of the mixture penetrates the filtering elements forming a permeate stream. In the ideal case, no cake is formed, but the material to be separated is enriched in the circulating phase, and a certain fraction is continuously being removed therefrom as a reject stream. The quality of the permeate is basically dependant on the pore size of the filtering element membrane. This pore size can be very narrowly defined. In microfiltration, the pore diameter can be 0.1-10 &mgr;m, and in ultrafiltration down to 1 nm. Thus, separation of molecules on the basis of size is possible by means of filtration. One application is the removal of pyrogens from water, pyrogens having relatively large molecules.
Membrane filtration elements have porous carrier structures, on the filtering surfaces of which membranes having well defined pore sizes have been formed.
Membranes with small pore sizes can be formed from e.g. polymer films. These are not, however, particularly resistant to mechanical and thermal stress, which is a drawback in sterilisation and cleaning. Ceramic membranes on porous, ceramic carrier phases have also been developed. Most ceramic materials resist heat in addition to being mechanically durable. On a ceramic element, as on other materials, surfaces can also be sealed, i.e. made impermeable. Thus, three types of surfaces may occur on a ceramic filter element: membrane coated, sealed, and untreated, through which permeate flows freely.
For example, in U.S. Pat. No. 5,104,546 a multilayer ceramic tubular filter element is disclosed, suitable for pyrogen removal by ultrafiltration. The end walls of the element as well as the inner walls of the channels are membrane coated. When the element is fitted into a shell or housing, and a mixture is pumped through the inner channels, permeate flows through the porous carrier to the shell side. Seals, for example rubber gaskets, must be provided between the shell and the element end walls.
In addition to tubular elements, plate-shaped ceramic elements are generally known, see for example WO 98/28060 and references therein. A plate-shaped filter element has a membrane coated outer surface and an aperture in the centre or elsewhere, the walls of which aperture are not membrane coated. Filters can be assembled by stacking such plates, as shown in e.g. GB 1 268 875. Due to the shape of the plates, the mating surfaces may be higher than the filter surfaces, whereby free filter area is provided between the plates as stacks are formed; plates may be separated by spacers; or if the seals between plates are thick enough, the seals function as spacers. Thus, the central apertures form a permeate retrieval and exit channel, and the plate stack forms a flanged-tube structure across which the pressurised mixture to be filtered is allowed to flow. The mixture thus flows on the shell side and the permeate is collected from the inner channel.
As stacks are assembled, seals must be used between the plates and/or the spacers, and between the elements and the housing, the number of seals being proportional to the number of plates used.
FIG. 1
shows a filter plate
1
according to the prior art, having a central bore
2
.
FIG. 2
shows a section of part of a stack formed from plates
1
. Four plates are shown in the Figure, but the stack may comprise additional plates depending on required filter surface area; the stack is confined in a housing not shown in the Figure. In the housing, a flow of pressurized mixture for filtration is maintained. The plates comprise a porous ceramic support body
3
, formed from e.g., aluminum oxide and having a relatively large pore size, e.g., >1 &mgr;m, and on the surface of the support body has been formed a ceramic filter membrane
4
. The membrane has been formed from e.g., titanium or zirconium oxide. The central bores
2
of the plates form a permeate channel
6
; in the embodiment shown, a perforated tube
5
is provided therein. The joints between the mating faces of the plates are sealed by means of O-rings
7
.
During the filtration process, bodies smaller than the pores of membrane
4
, as e.g. water molecules in the case of ultrafiltration, pass through the membrane due to the pressure difference, and are carried through the porous support body into the permeate chamber and exit through the end fittings.
In the above described filter units, the seals between the mixture and the permeate side are potential leak sites, and in demanding filtering processes a small leak may immediately render the permeate useless, as the mixture to be filtered penetrates into the permeate channel.
In tube or tube bundle type ceramic filter units, the sealing problem is associated with the end sections of the units. If the feed mixture penetrates the joint between the housing and the filter clement and enters the shell side, the permeate is contaminated. In plate stack-type filter units, potential leak sites are the joints between the plates and between the plate stack and the housing.
SUMMARY OF THE INVENTION
A main object of the invention is a crossflow-type membrane filtering unit wherein the sealing system between the filtering element and its housing comprises a channel through which tho feed mixture on penetrating the seal system can flow without penetrating the whole sealing system, thereby indicating a leak.
In an embodiment using plate-shaped filtering elements, two concentric seals are preferably used between the filter plates. Thus, a space delimited by the seals and two plates is formed. At the location of said space, the plate surface is sealed, that is made impermeable. In addition, at the location of said space, at least one opening is provided in each plate, interconnecting the spaces between adjacent plates in the stack. The walls of the opening(s) are also scaled, and consequently an isolated, sealed space surrounding the permeate channel is formed. A tube or equivalent can be provided to connect this space to the outside. If a leak occurs in a seal between two plates, the fluid may thus flow into the isolated space and out. Thus, a leaking seal can be detected immediately, and no possibility exists for the feed mixture to enter the permeate channel.
Preferably, the double seal system consists of two concentric, annular seals of different diameter. The seals may be of the O-ring type, and corresponding grooves may be provided in the filter plates. A huge range of O-ring seals designed for various chemical and physical conditions is available, and as the chemical and thermal durability of ceramic filter plates is usually high, a filter structure is thus provided which is functional in harsh conditions, with a low risk for permeate contamination by feed mixture.
In embodiments comprising filter elements of a tube or tube bundle structure, the joint between the filter housing and the filter element must be reliably sealed. The seal can be formed between the element end wall and the housing, or between the side walls of the element and the housing, and appropriate surface sealing of the filter element at the site of the seal system must be provided.
Surface scaling can be effected in various manners known to the person skilled in the art. On ceramic filter elements the surface may be glazed, or organic coatings may be used according to the process requirements. Organic

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