WO1979000765A1

WO1979000765A1 - Rotary drum filter and method of operation for medium cleaning

Info

Publication number: WO1979000765A1
Application number: PCT/US1979/000166
Authority: WO
Inventors: W Wegener
Original assignee: Envirotech Corp
Current assignee: Envirotech Corp
Priority date: 1978-03-15
Filing date: 1979-03-13
Publication date: 1979-10-04
Anticipated expiration: 1980-09-15
Also published as: ES478684A1; MX147795A; IT7921027A0; NL7901975A; CA1110552A; FR2419746A1; IT1192640B; AU4510679A

Abstract

A rotary vacuum filter wherein each filtration sector (11) of the filter is backwashed with filtrate and/or separately supplied gas or wash liquid while the sector is submerged in the feed slurry in the tank. As each sector starts to resurbmerge in the tank, vacuum is applied so that the sector and associated hydraulic system is quickly and completely filled with filtrate and a thin cake or film f solids forms on the filter medium (13) whereupon with the sector still submerged, vacuum is cut off and a positive pressure is applied by either or both pressured gas or liquid supplied through the valve (14) thus forcing filtrate back through the cloth to effect washing thereof. After the wash, vacuum is reapplied for cake formation. The drum (3) includes an internal manifold (17) under each filtration sector. The manifolds are mounted on the interior wall of the drum and are secured directly to the drum wall as structural members. Additionally, each manifold serves as a reservoir for storage of wash liquor.

Description

ROTARY DRUM FILTER AND METHOD OF OPERATION FOR MEDIUM CLEANING

TECHNICAL FIELD

This invention relates to rotary vacuum filters and in particular to improved ways and means for effective washing of the filter cloth without interrupting the filtration operation.

In operation of rotary filters, such as rotary drum or 'disc vacuum filters, as each sector of the filter rotates through feed slurry in the tank, filtrate Is.drawn through the filter cloth while solids are retained on the surface. After the sector emerges from the slurry, continued application of vacuum further dries the cake and a wash liquor may also be applied to displace the filtrate still in the cake. There¬ after, as the sector nears the point of resubmergence, vacuum is released and a positive pressure is applied to cause the cake to drop off the filter surface to discharge; then the sector is resubmerged to repeat the cycle.

BACKGROUND ART

Proper filter operation requires that the filter medium cloth covering the filter be kept clean. That is, it must be porous. In many operations, some of the solids enter into and plug or blind the pores of the cloth and eventually cause shut¬ down for cleaning or replacement of the filter medium. In filtration to separate solid wax from a mixture of crude oil and an extraction solvent such as methyl ethyl ketone, the mixture is cooled to solidify wax and the resulting cooled slurry is filtered. By its nature, wax is tenacious and ad tionally the crude oil contains many solid impurities. The result is that the filter medium blinds relatively quickly requiring frequent shutdown for cleaning. Such cleaning re quires a hot solvent hence the down time is considerable si the entire filter is heated and then recooled.

The best art known to applicant is U.S. 3,627,139 whic discloses the desirability of backwashing each cycle but ac nowledges the difficulty attendant thereupon, and suggests piping arrangement wherein filtrate is transferred from a filter sector that has emerged from the slurry to a sector entering submergence. The patent states the problem clearl but the solution is costly in terms of equipment and relati complex. The present invention solves the problem in a sim straightforward manner bythe means hereinafter described.

DISCLOSURE OF INVENTION It is the object of this invention to provide, in a ro vacuum filter, ways and means for reverse washing of the fi medium during each continuous filtration cycle. It is a further object to provide a method of filter operation for use in separating wax from cooled crude oil a solvent by which the filter cloth is kept in an unblinded_. c dition.

An additional important object is the provision of a d construction in which there is provided a manifold immediat _ underlying each filter sector which provides a reservoir of liquor (filtrate) within the drum between the filter surfac the valve. The wash liquor is thus rendered readily availa for cloth wash with minimum expenditure of energy. That is the filtrate used for cloth wash need not be drawn into a complex hydraulic system, but need travel only a short dist into a simple single compartment manifold or reservoir exte the length of the drum and communicating directly with the sector at spaced points therealong. The result is that whe

BAD ORIGINAL reverse pressure is applied, wash liquor (filtrate) need only be displaced directly from the manifold through the cloth.

An even further object is the provision of a drum con¬ struction in which the manifold is secured to the inside surface of the drum in such a manner that it is a structural part of the drum contributing to the strength thereof and eli¬ minating piping between the manifold and filter sectors on the drum surface.

In accordance with the invention, the filter valve is set so that as each sector initiates re-entry into submergence in the slurry, it is subjected to vacuum for a short time suffi¬ cient to fill the sector, and associated hydraulic system, in¬ cluding manifolds and conduits with filtrate then, while the sector is still submerged, a positive pressure is applied to force a portion of filtrate from the system back through the filter medium into the tank. Pressure for the backwash can be provided by either gas or liquid or. both supplied through the filter valve and hydraulic system. In special cases where blinding is especially stubborn, additional cleaning can be accomplished by supplying additional solvent to the cloth.

This reverse flow or backwash dislodges solids from the cloth. If additional gas or liquid is used, it is selected to be com¬ patible with the material being filtered. For instance,, in the separation of wax from crude and methyl ethyl ketone solvent, a clear solution of the solvent may be used. In the normal case where extra pressurized solvent is not required, it is an advantage of the invention that the backwash is accomplished at virtually no solvent cost. This is so because the liquid used for backwash is drawn from the slurry and is returned thereto without loss or dilution of the slurry.

In order to insure that a sufficient volume of filtrate is available for backwash, the filter is constructed with internal manifolds between the surface sectors and the valve. The proper volume of wash liquid will be determined empirically and the conditions of backwash adjusted accordingly.

As soon as the backwash is completed, the valve again sub¬ jects the sector to vacuum and it resumes normal operation whereby filtrate is drawn through the cloth while solids dep on the surface thereof and after emergence from the slurry t cake is washed, further dried and discharged.

In order that the invention may be more readily under- stood and carried into effect, reference is made to the acco panying drawings and description thereof which is offered b way of illustration and not in limitation of the invention, scope of which is defined by the claims which are also inten to cover equivalents. BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a rotary drum filter embodying the invention;

FIG. 2 is a front elevational view of a filter valve embodying the invention; FIG. 3 is a side sectional view of the valve shown in Fig. 2 taken in the plane of line 3-3 of Fig. 2 and looking in the direction of the arrows;

FIG.'s 4-8 are simplified sectional views of a single filter sector going through an operating sequence in which the filter cloth is backwashed pursuant to the invention;

FIG. 9 is a partial sectional view of the drum taken as looking lengthwise of the drum of Fig. 1 adjacent the surfac

FIG. 10 is an elevational view of a part of the drum surface of the drum of Fig. 9 looking in the direction of arrows 10, the drainage grid being omitted for clarity;

FIG. 11 is a partial sectional view of the structure of Fig. 10 taken as looking in the direction of arrows 11 of Fig. 10.

The drawings illustrate a solvent oil dewaxing filter adapted to separating solidified wax from a chilled crude oi and solvent mixture in which it was initially dissolved at higher temperatures. The basic filter (1) comprises a vat or tank (2) ; a cylin¬ drical drum (3) in the tank journalled for rotation therein by trunnions (4), on the drum and.bearings on the tank endwalls. A cover (7) closes the vat with the drum inside. Inspection can be made through any of the glass covered ports (8) . In operation feed slurry to be filtered is introduced into the vat (2) by a convenient inlet and a constant level is main¬ tained therein in accordance with standard practice. The surface of the drum is divided longitudinally extending divi- sion strips (9) and outwardly extending end flange (10) into a plurality of individual filtration sectors (11) . Each sector contains a drainage grid (12) overlain by a filter medium (13) held in place by the usual caulking. As the drum rotates, the sectors continuously pass into submergence in the tank where cake is formed on the drum surface while filtrate is pulled through the cloth. After cake formation,, the drum rotates out of emergence and may be subjected to further washing and drying, all under the influence of vacuum originating in a usual pump, not shown, and communicated to the filter sector through the valve (14) then through axial ports in the trunnion (4) , in¬ ternal conduit (16) , manifold (17) and ports (18) in the individual sectors (11) .

Vacuum applied to the valve (14) through the large flanged openings (21) is maintained continually in the large valve chamber (15) whence it communicates through the trunnion ports whenever those ports rotate into registration with the chamber (15). Thus, the ports are subject to vacuum whenever they are not covered by bridge blocks or directly connected to a separ¬ ate pressure zone such as shown at (23) . Fig.'s 4-8 illustrate operation of the filter through the wash cycle. As the leading side of sector (11) starts to sub¬ merge, port (24) at the trunnion registers with a small hole (26) in the bridge block which provided communication with the vacuum chamber (15) thus vacuum is applied through a conduit to the lead outlet (18) in the sector and the sector and asso¬ ciated piping commence to fill. Upon continued rotation, the trunnion lead port (24) passes the small opening (26) in th bridge block and vacuum is cut off but the trail port (27) the trunnion registers with another small hole (28) in the block and vacuum is then applied to the trailing outlets (1 of the sector until the trail piping is filled. Further rotation brings both lead and trail ports (24) and (27) int registration with the openings (29) which communicate to a pressure source through the pressure zone, (23) and conduit (31) . The application of pressure forces the thin cake off the drum surface and is continued until the preselected vol of filtrate in the hydraulic system is forced through the f cloth back into the tank. As noted, in the normal case, on a part of the hydraulic system contents need be used in the backwash to prepare .the cloth for another cycle. The volum of backwash is best determined empirically for a given plan Adjustments in the valve or pressure settings control durat and force of back pressure flow to thus regulate the backwa volume.

The filtrate backflow through the submerged filter clo is a dynamic and uniformly distributed wash which acts even across the entire sector. This method will be useful in an filtration operation where cake formation occurs quickly so there will be ample time to conduct backwash and to reform the cake after backwash. Once the backflow has been completed, continued drum rotation brings successive sectors into register with the large vacuum chamber to effect cake formation and drying pr • to discharge.

The manifold location and drum construction are best illustrated in Figs. 9-11. As shown, each sector (11) is p vided with lead and trail ports (18) adjacent its leading a trailing edges. As used herein, the words lead and trail h reference to the direction of drum rotation. As illustrate all lead ports of a single sector communicate with a common lead manifold (17) and all trail ports of a sector with a common trail manifold (17) . A single conduit (16) then

BAD ORIGINAL

OMPI connects each manifold with one of the ports in the trunnion which in turn connect through a valve to a vacuum source. This single pipe arrangement reduces the number of internal pipes and enables the use of larger pipes^' and ports in the trunnion thus enhancing hydraulic efficiency of the filter.

The manifolds are rectangular. Each manifold of a given sector is located as close as possible to the adjacent mani¬ fold of an adjacent sector with the junction therebetween, preferably directly under a division strip. Thus, the trail manifold of any sector is located next to the lead manifold of the next succeeding sector; and the centerline between the manifolds is under the division strip (9) between sectors. The manifolds are utilized as structural members for the drum by securing them along their length, either with a continuous weld or a series of tack welds to the interior wall of the drum. Also, the ports in the drum wall and those in the top wall of the manifold are in registration with each other and their peripheral edges are sealed together, usually by welding.

A plurality of both lead and trail ports (18) is provided in each sector. The ports are formed as elongated slots located as close as possible and parallel to a division strip. This insures presentation of maximum opening for liquid flow when the port is on the low side of the sector. Also, the slotted port is quite easy to seal weld to a corresponding port in the top wall of the manifold with which it registers to provide communication with the manifold.

Location of the slotted opening with respect to the mani¬ fold side wall nearest the division strip is important. As shown, one side of the slot should be close aboard the top edge of the manifold sidewall. This will keep one or more slots adjacent the lower edge of the sector at all times while the drum is out of the slurry, whether the sector is ascending of descending thus enhancing filtrate removal. The slot location is also an aid to cake discharge by gas blowback through the trail slot. This is so because when a sector is descending to the discharge level the trail port is at the top and the manifold acts to trap and retain residual or entrain moisture that would otherwise be carried back into the filte cake when pressured gas is supplied through the trail port f cake discharge. Further, location of the ports at the extre opposite sides of the sector insures sweeping of the complet sector during purge when positive pressured gas is introduce through the trail slot and withdrawn under vacuum through th descending lead slot. Complete sweeping is further enhanced by staggering the lead and trail ports with respect to each other as shown in Fig. 10.

The filter is provided with a drainage grid (12) secure between the division strips (9) and a filter cloth (13) suit ably secured in place by conventional caulking and wire windings. A usual deflector blade to guide discharged cake into a cake removal scroll or other conveyor is provided.

The configuration and physical location of the manifold are important for two reasons. First, they provide an impro drum construction because they are secured to the inner drum surface and there are no intervening conduits.- Secondly, an this is very important to the cyclic wash operation, the man folds provide a reservoir of wash liquor (filtrate) just adjacent the filter sector. This means that very little ene is wasted by drawing in the necessary wash volume and very little reverse flow is required to effect uniform flow of th necessary small volume of filtrate for cloth wash. Thus, it adjacent that use of the^" manifold as a wash liquor reservoir directly adjacent the filtration sector which minimizes back- wash cycle time and energy consumption.

INDUSTRIAL APPLICABILITY Although the invention has been described in connection with a rotary drum filter, it is not limited thereto as it m be utilized in other rotary filters, such as disc filters, wherein successive filter sectors are rotated into and out o submergence in feed slurry wherein vacuum is applied and cak is formed on the surface of the filter medium while filtrate is drawn therethrough.

The invention has primary application in dewaxing oper¬ ations where there is danger of rapid blinding of the cloth by the solids being separated. While the invention is capable of and has been described as occurring every cycle, it may be operated at a lower fre¬ quency as the need is determined empirically. In any event, the improved wash reduces down time, increases filtration rate, and increases cloth life. In the case of separation of wax from crude oil and solvent, reduction in shutdown reduces wear and tear on materials of construction because the extreme variations in temperatures between hot wash (180°F.) and normal operation (as low as -15°F.) are drastically reduced.

The invention is not limited to filters having both lead and trail ports, but is readily adaptable to various types of hydraulic systems. Also, in the illustrated embodiment, it is not necessary that both lead and trail systems be- used for backwash as either may be used alone. Nor is it necessary that both lead and trail systems be used simultaneously for backwash as it may be desirable to have them backflow in series to effect a prolonged wash. All such variations are within the scope of this invention.

Claims

1. In a rotary vacuum filter wherein a filter cloth is secured to overlie a drainage deck on the surface of a filter member journalled for rotation through a slurry in a tank, cak is formed as successive sectors of the filter are rotated into and out of submergence in the tank while vacuum is applied to the inside of the filter medium through a valve; and vacuum is released to effect cake discharge after emergence from said slurry, improved means enabling cleaning of said filter cloth, comprising means associated with said valve for application of vacuum to each sector as it starts to submerge in the tank and for continuing■said vacuum until the sector and associated pip connecting it to the valve are substantially full of filtrate, means operative while said sector remains submerged to discon¬ tinue vacuum application, means to apply pressure to said pipi to force a selected volume of said filtrate back through said piping and cloth into said tank, and means to immediately re- apply vacuum to the surface of said sector to reform a filter cake thereon while drawing filtrate therethrough.

2. Filter according to Claim 1 in which said filter includ a rotary drum the surface of which is divided into longitudina extending filter sectors and said piping includes for each sec at least one elongated manifold secured to the inner surface o said drum directly adjacent said sector and in direct communi¬ cation therewith through ports in said drum wall in registrati with ports in said manifold.

3. A filter according to Claim 1 in which said means for applying pressure to filtrate in said piping comprises pressurized gas.

4. A filter according to Claim 1 in which said means for applying pressure to filtrate in said piping comprises liquid under pressure.

OM

5. In the operation of a rotary vacuum filter of the type having a plurality of filter sectors covered by a porous filter medium and mounted so that adjacent sectors pass successively into submergence in a slurry wherein vacuum is applied to draw filtrate through said silter medium into the filter hydraulic system while causing solids to deposit on said filter medium then after emergence from said slurry, cake is discharged from said filter medium; the improvement comprising the steps of collecting a volume of filtrate in a reservoir in said hydraulic system adjacent to and in direct hydraulic communication with said sector, discontinuing vacuum and applying pressure to said filtrate while said sector is still in submergence, continuing said pressure until a selected volume of filtrate from said reservoir has been forced through the filter medium; and there- after applying vacuum to said filter sector while still in submergence.

6. A filter comprising a drum formed as a cylinder equipped with trunnions on opposite ends and adapted to rotate in a tank so that successive portions thereof rotate into and out of sub- mergence in said tank, said drum having its curved outer wall surface divided by longitudinally extending division strips into a plurality of filter sectors extending end to end there¬ of, a flange at each end of said drum closing the ends of said sectors, a filter medium overlying said filter sectors, a plurality of ports in one of said trunnions extending axially therethrough, a plurality of ports in said drum surface spaced apart along at least one side of said sector close to said division strip, at least one manifold extending the length of the inner wall of said drum located under one side edge of each of said sectors, ports in the top wall of said manifold in registration with said ports in said drum, means sealingly con¬ necting the peripheral edges of said ports in said manifold with the peripheral edges of said ports in said drum, said mani- fold being connected by a single conduit to a port in one of said trunnions, said manifold being secured directly to the inner wall of said drum, valve means to which said single

^■ U EΛtT

OMPI conduit connects, means included in said valve for selectiv applying vacuum and pressure to said conduit and manifold, means including means to sequentially initiate application vacuum to said pipe as said sector starts to submerge and t continue said vacuum at least until said manifold is filled with filtrate and said sector is submerged, then to discont vacuum and apply pressure to filtrate in said conduit to dis place at least part of the filtrate from said manifold thro said sector and said filter medium and then to reapply vacu to said conduit while said sector remains submerged.

7. A filter according to Claim 6 in which the drum has ports along both sides of each of said sectors close to sai division strips, a manifold is provided- under each side of sector and has ports in registration and sealingly connecte with said ports in said drum wall each of said manifolds be connected to a port in said trunnion by a single conduit, s manifolds are formed so that adjacent walls of adjacent man folds are straight, and said manifolds are secured directly the inner wall of said drum.

8. A filter according to preceding Claim 6 in which sai ports in said drum surface and manifold are formed as elong slots arranged parallel to said division strips.

9. A filter according to preceding Claim 7 in which sai ports are formed as elongated slots and in each sector said elongated slots along one side edge thereof are staggered w respect to those along the other side edge thereof.

10. The invention substantially as shown and described herein.

OMPI