CA1183463A - Rotary drum filter - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An improved rotary drum filter is described. The filter includes a liquid collection means (80) communicating with at least a plurality of fluid conduits (31). The col-lection means (80) collect and discharge residual liquid re-maining in the fluid conduits (31) during each rotation of the filter drum to decrease the erosive effects of the residual liquid on the filter cloth.

Description

;3 1 BA~KGROUND OF THE INVENTION

2 This invention relates to an improved continuous

3 rotary drum filter means. More specifically, the sub~ect

4 invention is related to a continuous rotary drum filter for the separati~n o petroleum liquids, such as lube oils, 6 from solids, such as paraffins.
7 The separation of mix,ures of compounds can be 8 accomplished by various unit operations including distil-9 lation, solvent extraction and fractional crystallization.
Fractional crystallization is particularly well-suited to ll the purification of many mixtures economically in a one-12 stage operation. Frequently, the cry~tallization ~peration 13 is combined with another unit operation to produce an 14 improved separation process. In petroleum processing, operations fractional crystallization is often utilized to 16 separate waxy compounds from the lube oil fraction.
17 Frequently, a solvent is added to the mixture to provide 18 more favorable conditions for crystal growth and to dilute 19 the resultant chilled slurry to thereby permit continuous oil-wax separation.
21 One type of filter particularly well suited to 22 fractional crystallization, in general, and wax-lube oil 23 separation, in particular, is the rotary drum filter. In 24 this type filter, a filter drum continuously rotates within a filter vat containing the mixture to be separated. In 26 one phase of the rotation, vacuum is applied to the filter 27 drum causing a major portion of filtrate to be drawn through 28 the filter cloth and fluid conduits, while the solids and 29 a minor amount of filtrate are trapped by the filter cloth and build up to form a cake. A liquid solvent wash liquid 31 is often then sprayed over the cake and drawn through the 32 filter cloth and conduits to remove additional quantities 33 of filtrate from the cake. During another phase of the 34 rotation, high velocity gas is passed outwardly through the fluid conduits to purge the li~uid filtrate and/or solv~nt 36 and break the filter cake away from the cloth. During this 37 purge cycle, liquid adhering to the walls of the conduits f~

1 may be directed against sections of the filter cloth by ~ the high velocity gas, there~y causing erosion of the fil-3 ter cloth. Filter cloth replacement necessitates complete 4 shutdown of the filter and is relatlvely expensive, since S rotary filters frequently are thirty feet or more in 6 length.
7 One method for minimizing the erosive effects of 8 the liquid on the filter cloth has been to have an elongated 9 purge cycle in which the lead fluid conduit in the direction of flow is evacuated while the lag conduit, or conauit im-11 mediately following the lead conduit, is purged with high 12 velocity gas. Liquid purged out of the lag pipe is 13 directed into the lead pipe before it can strike the filter 14 cloth. At a predetermined point in the rotation, the vacuum is discontinued and the purge gas and any remaining liquid 16 is directed outwardly. Extension of the time period during 17 which the combination of purge and evacuation is accom-13 plished would decrease the amount of liquid remaining in 19 the lag fluid conduit when evacuation of the lead fluid is discontinued. However, this would require a decrease in 21 the rotational speed of the filter drum and/or a decrease 22 in the other filter drum cycle times. Either of these 23 methods thus would decrease the filtration capacity of 24 the unit.
Yet another method of decreasing the erosive 26 effects of the liquid wouid be to use a thicker or more 27 erosion-resistant filter cloth. Use of a thicker filter 28 cloth would increase the filter cloth cost and also in-29 crease the tendency of the filter cloth to bow while also decreasing the filter capacity. In addition, the thicker 31 the filter cloth, the more difficult it is to remove the 32 wax fines from the cloth. Use of a more erosion-resistant 33 filter cloth may not be advantageous, since a more erosion-34 resistant cloth may not have desirable filter cake dis-charge characteristics.
36 Other methods for decreasing the amount of liquid 37 blown from the fluid conduits onto the filter cloth have 3~63 1 been directed at the addition of liquid storage chambers 2 by the use of a series of capped pipes or the like attached 3 to the fluid conduits to trap the discharged liquid.
4 United States Patents 2,321,230; 3,262,575; and 4,032,~42 all are directed at individual liquid chambers attache!d 6 to each fluid conduit~ These devices have several dis-7 advantages. Since the volume of residual liquid which may 8 remain in each fluid conduit may be significantt these 9 fluid chambers each should have a large capacity to trap all the liquid discharged. However, entrapping this 11 residual liquid may result in undesirable deflection of the 12 chambers unless relatively complex support mechanisms are 13 provided. In addition, venting o~ the aforementioned 14 devices may not be sufficient, resultins in incomplete drainage of the chambers during filter drum rotation. In 16 addition, the aforementioned devices may not be easily 17 retrofitted to rotary drum filters presently in use.
18 Accordingly, it is desirable to produce a 19 modified rotary filter in which erosion of the filter cloth by entrained liquid is reduced or eliminated without 21 decreasing the ~ilter throughput or increasing the cost of 22 replacement filter cloths.
23 ` It is also desirable to have a rotary filter 24 design which effectively traps residual liquid and which is relatively inexpensive and reliable.
26 It is also desirable to have a rotary filter de-27 sign which is easily adaptable to rotary filters currently 28 in use for removing entrained liquid.

The subject invention is directed at an improved 31 rotary filter of the type comprising:

32 (a) a filter vat adapted to contain a filterable 33 slurry;
34 (b) a rotatable filter drum at least partially disposed in said filter vat and adapted to accept a filter 36 cloth, said drum having circumferential openings therein in 37 substantial alignment with fluid conduits disposed in said 38 drum;

.

i3 1 ~c) means to rotate said drum in said filter 2 vat;
3 (d) fluid conduits disposed in said drum in sub-4 stantial alignment with ~he circumferential openings in said drum, said conduits communicating with a valve means 6 and with a liquid collection means; and 7 te) a valve means communicating with said fluid 8 conduits for xegulation of the fluid 10w through said 9 conduits and through the circumferentially disposed openings in said filter drum, the improvement which comprises a 11 vented liquid collection means communicating with a 12 plurality of said fluid conduits, whereby,during one phase 13 of the rotation of said filter drum,liquid is directed from 14 said fluid conduits into said liquid collection means, and, during another phase of the rotation of said filter 16 drum, the liquid is removed from said liquid collection 17 means. In a preferred embodiment of the subject invention 18 the liquid collection means comprises a pair of liquid 19 collection means, one being disposed at each end of the filter drum. Each collection means comprises a series of 21 circumferentially disposed chambers, each chamber com-22 municating with the terminus of a lag fluid conduit and 23 having a venting means.

Figure 1 is an exploded isometric drawing of a 26 typical rotary filter incorporating the subject invention.
27 Figure 2 is an enlarged simplified isometric 28 view showing fluid conduits and the liquid collection means 29 of the subject invention.
Figure 3 is a simplified cross-sectional view of 31 a rotary filter indicating the operations performed during 32 each rotation of the drum.
33 DETAILED DESCRIPTION OF T~E INVENTION
34 Referring to Figure 1, an exploded/ simplified isometric assembly drawing of a typical rotary drum filter 36 is shown. The filter includes a filter vat, generally 37 indicated as 10 and a filter drum generally indicated as 1 20. Vat 10, having a substantially semicylindrical shape 2 is supported by columns 12 above base 14. An inlet 16 3 communicates with vat 10 and with the source of the slurry 4 to be separated (not shown). A level controller (not shown) usually is added to control the level of slurry in 6 vat 10 by regulating valve 18 in inlet 16. A filter hood 7 90 having an overall semicylindrical shape has a plurality 8 of perforated spray headers or drip pipes 92 disposed 9 therein parallel to the axis of rotation of drum 20 to wash the wax cake formed in the drum as hereinafter 11 described. Filter drum 20 is of a generally overall 12 cylindrical shape having a diameter slightly less than that 13 of the diameter of vat 10, such that drum 20 is rota-table 14 about its axis ~n vat 10 on a pair of trunions 22.. A
series of uniformly spaced channel sections 40 extenl 16 outwardly from the curvilinear surface of drum 20 parallel 17 to the axis of rotation. Terminal sections 42 disposed 18 near the ends of drum 20 connect each section 40 to an 19 adjacent channel section, to thereby define a series of sgements 44 circumferentially disposed around drum 20.
21 Each segment 44 has two series of circumferential openings 22 through drum 20 evenly spaced along substantially the 23 entire axial length of each segment to form circumferential 24 rows with one series of openings 32 in general alignment with fluid conduit 30 and with the other series of openings 26 34 in general alignment with fluid conduit 31. In each 27 segment 44 the leading opening 32 in the direction of 28 rotation hereinafter is referred to as the lead opening 29 while the immediately adjacent trailing opening 34 is referred to as the lag opening. Similarly, fluid conduit 31 30, communicating with opening 32, is referred to as the 32 lead fluid conduit, while fluid conduit 31 communicating 33 with lag opening 34 is referred to as the lag fluid 34 conduit. One of the pair of trunions 22 has a plurality of holes each of which communicates with lead and lag 36 fluid conduits 30, 31, respectively and with trunion valve 37 means 24 to perform varying operations during each revolu-.., 1~ ~3~

1 tion of drum 20 as described hereina~ter. The other 2 trunion communicates with a conventional drum rotation 3 means (not shown) to rotate drum 20 in vat 10. Each con-4 duit 30, 31 extends along substantially the entire axial length of d~um 20 terminating in a liquid collection means 6 80.
7 A means for removing the filter cake from filter 8 cloth 60 such as doctor knife 100 disposed in a compart-9 ment in vat 10 is shown discharging the filter cake into a screw conveyor 102 which directs the filter cake out of 11 vat 10 through outlet 104. Within each segment 4~ is dis-12 posed a filter grid 50 to maintain filter cloth 60 spaced 13 apart from drum 20. Filter cloth 60 is stretched over 14 channel sections ~0 and grids 50 and is retained in posi-tion by caulking bar 70 compressingly inserted between 16 channel sections 40 and by circumferential wires (not 17 shown).
18 It has been found that the liquid remaining in 19 lag conduit 31 tends to be blown to the ends of the conduit when blow gas is passed outwardly through the conduit while 21 flow to conduit 30 simultaneously is blocked by means of 22 trunion valve 24. Accordingly, liquid collection means 80, 23 shown in Figure 2, somewhat simplified for clarity, should 24 be in communication with at least a plurality of lag conduits 31, preferably in communication with a majority of 26 lag conduits 31, and more preferably in communication with 27 all lag conduits 31 to decrease the erosive effects of 28 residual liquid on filter cloth 60 and to prevent residual 29 liquid from being blown back into the filter cake. Col-lection means 80 preferably is disposed within drum ?0 in 31 communication with the termini of lag fluid conduits 31.
32 Although it is within the contemplation of this invention 33 that only one collection means 80 is utilized, it is 34 anticipated that a plurality of collection means typically will be used, preferably having a substantially annular 36 or toroidal configuration. In the preferred embodiment 37 described herein, a pair of collection means 80 having an 1 overall toroidal configuration are utilized, one disposed 2 in drum 20 adjacent to each drum wall 26. Collection 3 means 80 may ~e a separate body mounted in drum 20, or it 4 may be ~ormed by par~itioning the internal area adjacent to wall 26 of drum 20. Where existing filters are to be 6 adapted to use of the subject invention, there may not be 7 sufficient room between the outermost circumferential rows 8 of openings 32, 34 and drum walls 26 for collection means 9 80. In that event, a sufficient number of rows of cir-cumferential openings adjacent to each end may be blocked 11 off to provide sufficient room for installation of co].-12 lection means 80, and lag conduits 31 may be modified to 13 communicate with collection means 80 as hereinafter des-14 cribed.
The overall configuration of collection means 80 16 will be dependent upon the design of the filter means, the 17 required liquid storage capacity in the collection means, 18 and the space available within the filter drum 20 for 19 installing the collection means. To simplify construction, collection means 80 preferably is of an overall annular 21 shape, more preferably of an overall toroidal configura-22 tion.
23 Fiyure 2 shows collection means 80 comprising a 24 generally toroidal vessel 81 having individual chambers 82, each chamber separated from the adjacent chamber by a 26 partition 86. Each chamber 82 communicates with a 27 ter~,inus of lag fluid conduit 31 through chamber opening 28 88. Each chamber 82 also is provided with a vent means 29 such as vent conduit 84 to permit alternate gas and liquid residence in each chamber 82 during each revolution of drum 31 20. Chambers 82 should be sized such that each chamber 32 will contain all the r~sidual liquid discharged through the 33 respective terminus of lag fluid conduit 31 during each 34 revolution by the blow gas. It may be desirable to brace vessel 81 to drum 20 by radially extending supports 100 36 communicating with vessel 81 and drum 20.

, 3~ ;3 1 The m~thod selected for venting chamber 82 is not 2 critical provided that the venting is sufficient during all 3 phases of filter operation. Adequate venting is particular-4 ly important in allowing gas to displace liquid from each chamber when the chamber is submerged in the slurry. One 6 particularly effective method for venting individual 7 chambers 82 is to connect each chamber, such as charnber 82A, 8 to the generally diametrically opposite chamber such ~s 9 chamber 82F, via the vent conduits 84 joined at 85,although other means for venting chambers 82 also may be effective.
11 In this Figure, sections of all lag fluid conduits 31 and 12 all but two vent conduits 84 have been omitted for clarity.
13 A typical filter for the separation of wax from 14 lube oil has a filter drum 20, thirty feet in length and eleven and one-half feet in diameter. This dxum has an 16 effective filtering area of approximately 1000 square feet 17 with thirty lag fluid conduits 31, 2 1/~ inches in diameter.
18 In this embodiment, liquid collection means 80 may comprise 19 two toroids fabricated from eight inch O.D. schedule 40 type 316 stainless steel approximately nine feet in dia-21 meter. The rings are partitioned into 30 substantially 22 equivalent chambers 82, each chamber communicating with the 23 terminus of a lag fluid conduit 31 and with the diametrical-24 ly opposite chamber via vent conduit 84, which may be 25 fabricated from small diameter thin-walled metal tubing, 26 such as half inch diameter stainless steel tubing.
27 Preferably tubing which is somewhat flexible should be 28 used, since all vent conduits 84 will pass near the center 29 of vessel 81.
The sectors of Figure 3 illustrate the varying 31 operations performed simultaneously by a typical rotary 32 filter such as that of Figure 1, which incorporate the sub-33 ject invention. When a point on filter cloth 60 is in a 34 position included within sector A, that point is under 35 vacuum. Filtrate is drawn through filter cloth 60, cir-36 cumferential openings 32 and 34 and fluid conduits 30 and 37 31, respectively, into a filtrate storage facility (not 3~63 1 shown). The solid wax crystals are ~etained on filter 2 cloth 60 and buildup to form a wax filter cake. Towards 3 the end of this phase, blow gas passes from chamber 82F
4 through vent conduit 84 into chamber 82A to pressurize

5 chamber 82A momentarily, thereby forcing liquid in chamber

6 82A out of the chamber through lag conduit 31 int~ filtrate

7 stoxage. As each point on filter cloth 60 rotates to

8 sector B, that point rises out of the slurry. Vacuum

9 continues to be applied to sector s to continue to draw any

10 remaining filtrate through circumferential openings 32 and

11 34 and fluid conduits 30 and 31, respectively, thereby

12 further drying the filter cake. Any remaining liquid in

13 chamber 82A is forced from the chamber through lag conduit

14 31 by the combination of the pressure in chamber 82A and

15 the vacuum in the lag conduit. When the point rotates

16 still further to a position within sector C, valve means

17 24 continues evacuation of fluid conduits 30 and 31, while

18 a wash fluid is sprayed through spray headers 92 in filter

19 hood 90 to wasn filtrate from the wax cake. At least a

20 portion of the filtrate remaining on the wax cake as well

21 as wash fluid passes through the wax cake, filtex cloth 60,

22 circumferential openings 32, 34 and fluid conduits 30, 31,

23 respectively, for removal from the filter. The wash fluid

24 generally comprises a liquid which is miscible with the

25 filtrate but not with the solids of the filter cake. In

26 the separation of lube oil from wax, this wash liquid

27 frequently is a mixture of ketones or ketone/toluene. When

28 the point on filter cloth 60 rotates to the position shown

29 by sector D, the filter cake is again dried by discontinu-

30 ing external wash flow and continuing to apply vacuum to

31 sector ~ which pulls wash from the wax cake through cir-

32 cumferential openings 32, 34, and fluid conduits 30, 31,

33 respectively. As the point rotates still further to sector

34 E, purge gas, typically having a velocity of about 100 feet/

35 second, is supplied to lag fluid conduit 31 communicating

36 with lag opening 34 while the i~mediately adjacent lead

37 opening 32 continues to be maintained under vacuum. This i3 1 serves to cause some of the liquid remaining in lag fluid 2 conduit 31 communicating with lag opening 34 to flow into 3 fluid conduit 30 commllnicating with lead opening 32. As 4 the point rotates still further to sector F, lead opening 3~ is no longer evacuated. The purge gas which is called 6 blow gas at this point, forces at least a portion of the 7 rPmaining liquid in fluid conduit 31 into a chamber 82F
8 of 'iquid collection means 80~ Blow gas also passes through 9 vent con~uit 84 to pressurize the diametrically opposite chamber 82A as previously described. Blow gas also contacts 11 filter cloth 60, causing the cloth to billow out slightly, 12 thereby facilitating the subsequent removal of the cake by 13 doctor knife 100. It should be noted that if li~uid col-14 lection means 80 were not installed in the subject filter, liquid remaining in conduits 31 would be conveyPd ~y the 16 high velocity purge gas against filter cloth 60 causing the 17 cloth to erode, particularly at the ends of drum 30 where 18 the liquid tends to collect.
19 The relative length of phases A-F is shown in Figure 5 for a typical filter. It is to be understood that 21 the length of time that any given location on filter cloth 22 60 spends in a particular phase may be ad~uste~ by varying 23 the size of the sector devoted to that phase and by varying 24 the rate of rotation of drum 20. In addition to decreasing the erosion rate of the filter cloth, use of the subject 26 invention also may permit a decrease in the time required 27 for phases E and/or F, thereby permitting an increase in 28 the filtration time in each revolution.
29 While the invention has been described with respect to a specific embodiment, it will be understood 31 that this disclosure is intended to cover any variations, 32 uses and adaptations of this invention including such 33 departures from the present disclosure as come within 34 known or customary practice in the art to which the in-vention pertains and as fall within the scope of the in-36 vention.

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a rotary filter means of the type com-prising:
(a) a filter vat adapted to contain a filter-able liquid-solid slurry;
(b) a rotatable filter drum at least partially disposed in said filter vat and adapted to accept a filter cloth, said drum having circumferential openings therein in substantial alignment with fluid conduits disposed in said drum;
(c) means to rotate said drum in said filter vat;
(d) fluid conduits disposed in said drum in substantial alignment with the circumferential openings in said drum, said conduits communicating with a valve means and with a liquid collection means; and (e) a valve means communicating with said fluid conduits for regulation of the fluid flow through said conduits and through the circumferentially disposed openings in said filter drum; the improvement character-ized by a vented liquid collection means communicating with a plurality of said fluid conduits, whereby during one phase of the rotation of said filter drum, liquid is directed from said fluid conduits into said liquid collection means, and, during another phase of the rota-tion of said filter drum, the liquid is removed from said liquid collection means.

2. The rotary filter means of claim 1 further characterized by the fluid conduits comprising lead fluid conduits and lag fluid conduits and further characterized by said liquid collection means communicating with at least a plurality of the lag fluid conduits.

3. The rotary filter means of claim 1 above further charac-terized by the liquid being removed from said collection means through said fluid conduits.

4. The rotary filter means of any of claims 1-3 herein-above further characterized by said liquid collection means com-prising a pair of collection means, one disposed adjacent to each end of the filter drum and further characterized by each of said liquid collection means communicating with at least a majority of said lag fluid conduits.

5. The rotary filter means of any of claims 1-3 herein-above further characterized by each of said liquid collection means including a generally annular vessel having a plurality of circum-ferentially disposed chambers therein.

6. The rotary filter means of any of claims 1-3 herein-above further characterized by said vessel being substantially toroidal having a plurality of circumferentially disposed chambers therein each of said chambers communicating with a terminus of a lag fluid conduit.

7. The rotary filter means of any of claims 1-3 herein-above further characterized by each of said chambers communicating with a terminus of a lag fluid conduit and further characterized by said vented liquid collection means including venting means communicating with each of said chambers.

8. The rotary filter means of any of claims 1-3 herein-above further characterized by each of said venting means communi-cating with said venting means for a generally diametrically opposed chamber, whereby blow gas injected into one chamber passes through said chamber and through said venting means for said generally diametrically opposed chamber to vent said generally diametrically opposed chamber.

9. The rotary filter means of any of claims 1-3 herein-above further characterized by said rotary filter means including a filter cloth disposed over the circumferential openings on the filter drum, and means for removing filter cake formed on the filter cloth, whereby, during one phase of each rotation of the filter drum slurry is drawn onto the filter cloth with at least a portion of the filtrate passing through the openings into the fluid conduits while solids form a filter cake on the filter cloth, and during another phase of each rotation, the filter cake is removed from the filter cloth.