CA1046947A - Rotary filter wash distributor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A wash liquid distributor for a rotary drum filter which distributes liquid uniformly across the filter cake and provides very effective washing.
The distributor apparatus in its preferred embodiment comprises an elongated hollow pipe, one or more being used in parallel operation, each pipe having a multiplicity of evenly spaced and axially aligned small holes. Each hole imposes a substantial pressure drop on the fluid passing therethrough and creates a uniform distribution of the wash liquid along the distributor pipe. To avoid damage to the filter cake from the jets of liquid produced, a diffusion channel is provided which damps the kinetic energy of the jets and subdivides the liquid entering the channel. Upon leaving the diffusion channel the liquid is collected and redistributed from drip points spaced at intervals which are generally narrower than those of the holes in the distributor pipe. In the preferred embodiment such drip points are provided by helical overwrap of wire which serves both to collect liquid leaving the diffusion channel and to redistribute it onto the filter cake. The three-step procedure of the distributor creates a multiplicity of narrow pools of liquid on the filter cake which provide efficient washing when drawn through the cake by vacuum inside the drum and avoiding channeling through the cake or blinding of its passages.

Description

6~3t47 ~Ci~G1~0UND OF Ll-~ I~TION
Rotary filters have been wldely used in the chemical industries for many years. It is typical of such filters that the solid particles which are laid down on the filter drum must be ~ashed in order to remove residual llquid remaining therein. It has been found extremely difficult to apply wash liquid satisfac-torily and consequently, such efforts have been the subject of much prior art.
An early means of disposing of wash liquid on a filter cake consisted simply of a trough suspended above a rotary filter drum with liquid overflowing a curved lip on one side of the trough. This simple technique would appear to be satisfactory, but in iact is found to be subject to substan-tial difficulties owing to the need for absolute leveling of such distribution troughs, especially when they are of substantial length. For example, a co~ercial filter drum may be 15 or 20 feet in length, so that only a slight deviation from the horizontal position would cause great disparity in wash rate from one end of the ca~e to another.
An improvement on the simple trough comprised an overflow trough which has been provided with V-shaped notches to create a weir-type distributor. Although the weir device will overcome some of the disadvantages of the overflow lip of the ear-lier patent, nevertheless it remains sesitive to the horizontal leveling of the trough.
For some purposes such trough systems may provide a satisfactory result. It may be inferred from ' the prior art, however, that improved distribution was required since more complex belt distributors were in-troduced. These de-vices have as their objective uniform distribution of wash liquid by passing it through a porous belt, whereby the liquid is spread out onto the cake to provide a thin film of liquid. At the same time, to assure uniformity in the wash rate, the cake was leveled by the belt. While such devices would seem to be satisfactory for some materials, if the cake is subject to blinding by scraping it, it is quite probable that a belt distributor would cause a reduc-tion rather than an improvement in wash Iiquid distribution sincethe passages in the cake could be blocked by the rubbing action of the belt.
Recently, a distribution system for a rotary filter i.n a process similar to that for which the present invention was developed has been disclosed. In that system, distribution pipes are fed with liquid from several entry points and the liquid flows outwardly through small holes in the pipes and runs down and around groovillg and drips off the bottom of the pipe. In such a distribu-tor only a very small pressure can be used since the liquid must flow around the pipe rather than jetting out of the small holes.
Thus, since very little pressure can be applied to the distributor pi.pe, it is not possible to obtain optimum distribution along the pipe and distribution will be disturbed by changes in wash flow rates. The process to which this recent development was directed is a process for removal of wax crystals from lubricating oils. In that process, oil "

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is dissolved in solven-t and chilled to form wax crys-tals which are then removed by filtration. As will be appreciated, the wax crystals are soft and consequently the spaces between the crys-tals may be easily blinded by physical contact. As a result, belt distributor systems are unsatisfactory since they would have a tendency to blind the cake and thus create uneven distribution through it. If the wash liquid leaves the distributor pipes at a high velocity, it can impact against the cake with sufficient force -to dislodge it or to cut channels in it and thereby pre-vent uniform washing.
Such filters can also be provided with spray nozzlesmounted on distributor pipes. Since spray nozzles crea-te rela-tively high velocity sprays, reasonably even distribution of liquid i9 possible at the outlet of the spray nozzles. However, the high velocities with which the spray issues have detrimental effects on the cake porosity and are undesirable. At the same time wash rates can vary widely depending on the nature of the wax crystals.
When this occurs, the shape of the spray will change with the liquid pressure and coverage by the spray nozzles will be depen-dent upon the wash rate. ~ccordingly, at low wash rates poorcoverage of the cake and poor washing often occurs.
The present invention has addressed itself to the problems whiGh have been discussed. In the wax removal process - . - . : :
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~L~46~L7 which is described generally in U.S. Patents 3,773,650 and3,779,894 rela-tlvely high wash rates are possible. The crystals which are pxoduced by this wax producing process are substan-ti.ally spherical in nature and consequently the filter ' ~ ' .

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cake is more porous than in prior art processes and thus the filter cake can and should take a siynificantly higher wash rate. Thus, it has become even more important that hereto-fore to obtain a uniform distribution of wash without impart-ing higil velocities to the wash liquid which could cause damage or blindiny of the cake. The present invention has satisfactor-ily solved these problems and is disclosed in the detailed des-crip-tion which follows.
SUMMARY OF THE INVENTION

The wash distributor of the inven-tion provides good distribution of wash liquid onto a filter cake at varying flow rates and permits proper adjustment of the wash rates to suit the feedstocks being processed. In addition, distribution occurs in the form of a multiplicity of continuous narrow pools of ]iquid which are laid down with a minimum disturbance of the porosity of the cake. Application of the wash liquid to the cake takes place in a three-step process. First, a uniform distribution of the wash liquid a:long the distributor pipe is provided by using a higher than usual pressure and creating a multiplicity of jets exiting from small holes axially spaced along the pipe and facing away from the filter cake. There-after, the kinetic energy of the jets is damped out in a dif-fusion channel adjacent to the jets and extending around -the exterior of the pipe. Once the kinetic energy of the jets has been damped out and the liquid further diffused, it is collect-ed and redistributed onto the filter cake from a series of drip points. In the preferred embodiment, the collecting and distri-~ '.
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~114~9~7 buting ~unction is providecl by a helical wire overwrap. Li~uid leaving -the distribution wires falls as a series of thin streams whlcll tend to break up into small droplets prior to contact-ing the fi.lter cake. ~pon contacting the fil-ter cake pools ~ .

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of liquid are formed a-t the poin-t of impact and then join to foxm a continuous uniform narrow pool of wash liquid ex-tendiny completely across the filter cake, thereby creating a condition highly suitable for efficient washing. The improved washing characteristic of the present invention is shown in a wax re-moval process where the wax cake contains 20 to 30~ lower residual oil content than when spray no2zles are used.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a perspective view of a single wash distribu-tor of the present invention.
Figure 2 is an enlarged view of a portion of the distributor of the invention.
~ igure 3 is a sectional view taken substantially along line 3-3 of Figure 2.
DESCRIPTION OF THE PREFERRED EMBODI~ENT
The ideal method of providing a wash liquid to a rotary vacuum fil-ter cake would be to lay down, with essentially no impact, a uniform layer of wash liquid at a predetermined position on the filter cake. Following the first layer, addition-al layers should be deposited in sequence as required to effectthe desired washing. Any disturbance of the surface of the filter cake is considered undesirable since it leads to cake blinding of the passageways between the particles and, in extreme cases, even ~ - 6 -.
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~6~ 7 dislodgment of the cake from the drum. It should be noted that a controlled amount of wash is required. While the disadvantages of insufficient washing are clear, that is, residual liquid re-mains behind in the filter cake, if excess wash liquid is used, greater than that which the filter cake can accept, the excess wash runs off the cake and into the bottom of the filter where it dilutes the feed. Thus, it is important to . .~ . .

' maintain the desired wash flow rate and -to place the liquid only on the cake.
Reference to the prior art patents and in particular to~U.S. 3,729,~1~ illustrate the general arrangement of a rotary vacuum filter. In the perspective view of Fi~ure 1, the mode of deE~ositing the wash liquid by the present invention is illustrat-ed. Liquid enters the distributor pipe 10 throu~h inlet 11 under a pressure of at least 3 psig. It leaves the distributor 10 in the form of a care:Eully controlled series of streams 12 fallin~
only under the pull of gravity and sufficiently finely divided so that the streams 12 tend to break up into fine droplets 12a prior to reaching the surface of the cake 13. After being de-posited on the cake with minimum impact force, the droplets re-combine to form pools of liquid which spread to join adjacent poolsl resulting in the appearance illustrated in Figure 1, that is, of a continuous narrow pool of li~uid 1~ deposited across the length oE the filter cake 13. If a plurality of distributors is used, as would typically be the case, the cake would appear to be striped with the plurality of deposits of wash li~uid. It is characteristic of the distributor that this appearance can be obtained over a wide variation of flow rates without any distur-bance of the surface of the cake itself. A typical wash rate would be approximately 0.2 to 2.0 gallons per minute per linear foot of filter cake surface.

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~6;P4~9~7 The distribu-tor 10 comprises a pipe 15 with an over-lay of diffusing material 16 covered by an outer solid wrap 17 to create a diffusion channel 19 between the inner pipe 15 and the outer wrap 17. The outer wrap 17, which would "'.,'.
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6~7 normally be of a sheet metal, but could be plastic or other material, does not extend fully around the pipe. It is secured to the pipe 15 beneath by means of a helical wire wrap 18 which has turns spaced approximately 1/2 inch apart, securing the over-wrap 17-and at the same time collecting and redistributing liquid passing ou-t of the diffusion channel 19 between the pipe 15 and the overwrap 17. Although the wire serves a dual purpose and is convenient, other means of securing the overwrap could be used, with collec:tion and redistribution pOilltS being provided by some other means, for example, clamps having a grid for redistribution positioned between them. In a typical large commercial filter, the wire wrap has a diameter of approximately 1/8 inch.
Figure 3 illustrates more clearly the function of a preferred embodiment of the invention. Wash liquid passes into the main distributor pipe 15 under pressure and then jets out at relatively high velocity through the uniformly spaced, axially ali~ned holes 15a. The large pressure drop assures better distri-bution of the liquid than if only a small pressure is used, as in ~ the prior art~ In a typical commericial application these holes may be of the order of 0.015 to 0.025 inches in diameter. They are typically spaced 1 inch apart and directed away from the fil-ter cake. It will be appreciated that if a pressure drop of about 15 pouncls per square inch is taken across such a hole the result would be a substantial jet of liquid issuing from thè hole and impinging on the ~ilter casing from which it would splatter and not contact the filter cake pxoperly for good washing. Of course, .

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~L69~7 such jets directed in a downwardly direction toward the filter cake would tend to penetrate the cake and cut it so as to dis-lodge the cake or at the very least create a channeling effect which would cause the wash liquid to bypass the cake.

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6~7 To avoid both of -these problems, the jets thus are turned away ~rom the filter cake and the kinetic energy created by the distribution process is damped and the liquid is distributed through a ~ifEusion channel 19 formed between the distributor pipe 15 and the overwrap 17. The spacing between the distribut-or pipe 15 and the overwrap 17 is about 1/16 inch, creating a narrow channel through wilich the liquid passes. The diffusion channel 19 performs its intended function by means of a relative-ly coarse wire screen 16 located between the distribution pipe 15 and the overwrap 17, so that the edge of the screen faces the flow of liquid. Although a wire screen 16`has been found to perform quite satisfactorily in commercial operation, other means of di:Efusing the liquid jets could oE course be applied.
It would also be within the scope of the invention to utilize larger holes which would be less effective in dis-tributing liquid and to replace the screen used in the dif-fusion channel with a device which would require a higher pres-sure drop and thus provide both a diffusing and a distributing function. Sintered metal, for example, might be chosen for this purpose.
It is typical that the overwrap 17 covers appro~i-mately 270~ of the 360 available, leaving about 90 open at ; the bottom for collection and redistribution of the liquid. It will be appreciated that the liquid issuing from the diffusing ~ 9~

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channel 19 has little or no kinetic energy. Accordingly, its distributi.on onto the eake below would be of a random nature since it would collect at varying poin-ts along the outlet of the ehannel and drip off onto the cake below. In order -to obtain a uniform distribution, the liquid moving at relatlvely low velo-eities must be colleeted and distributed in order to assure a uniform di.stribution on the eake 9~
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~6~7 below~ Accordingly, the wire overwrap 18 is used which has been discussed previously. The function of the collecting wires 18 is to accept the liquid which leaves the diffusion channel 19 and to provide uniformly distributed drip points. In a typical commercial filter the distribution holes 15a are spaced approxi-mately one inch apart, whereas the wire overwrap is spaced about 1/~ inch apart, which gives satisfactory redistribution. Some variation ln the spacing would be possible without departing from the spirit of the invention. Low velocity liquid leaving the diEfusion channel 19 is collected on the redistributing wires 18 until sufficient wash has been collected so that it runs off by the force of gravity. Typically, with the number of wires that are used, the stream is extremely small and while it runs off as a continuous stream 12, it fractures into multiple droplets 12a prior to reaching the filter cake which is ordinarily about ~-8 inches below. Thus, it will be seen that a minimum force is di-rected against the filter cake by these droplets. Since the fil-ter cake cannot accept them instantly at the ra-te at which they are deposited on the cake, the droplets recombine to form a pool which then extends to join adjacent pools and the net effect achieved is the narrow pool which has been heretofore discussed and illustrated in Figure 1.
The effectiveness of the distributor of the invention is clearly demonstrated by the results of the wax removal process where a 20~30% improvement in residual oil content was achleved Gompared to prior art spray nozzles.

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The three-step dis-tribution process which has been described is carried ou-t successfully by the distributor illus-trated and discussed herebefore. Some variation in the specific construction details are possible without e~ceeding the bounds o:E the invention which is defined by the scope of the claims which follow.

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Claims (11)

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

1. A liquid distribution method suitable for distributing wash liquid over cake retained on a rotary filter drum, comprising the steps of:
(a) passing liquid under a predetermined pressure into a distributor pipe;
(b) discharging said liquid from a plurality of holes in the distribution pipe at a relatively high velocity into a diffusing channel wherein the liquid is substantially uniformly distributed by diffusing means;
(c) collecting said liquid from the diffusing channel and substantially uniformly distributing it at a relatively low velocity to points located along the length of the pipe; and (d) discharging said collected and uniformly distributed liquid from said pipe in the form of substantially small continuous streams which fracture into droplets for deposit onto said cake on said filter drum at a rate of deposit such that said droplets recombine on said cake to form narrow pools of liquid.

2. A method according to claim 1 further characterized by including the step of passing said wash liquid into said pipe at a predetermined pressure of at least 3 psig.

3. A method according to claim 1 or claim 2 further characterized by including the step of providing multiplicity of said pipes in parallel and performing steps (a) through (d) for each of said pipes.

4. A wash liquid distributor-adapted for use with a rotary filter drum comprising:
(a) an elongated hollow distributor pipe which is connectible to be supplied with wash liquid, the said pipe having a plurality of holes for the passage of wash liquid from the interior to the exterior of the pipe;
(b) liquid diffusing means overlying said holes and partially surrounding said pipe;
(c) a solid cover partially surrounding the pipe around the liquid diffusing means and defining, with the exterior of the pipe, a diffusion channel wherein said liquid diffusing means serves, during operation, to distribute liquid substantially uniformly in said channel; and (d) overwrap means securing the solid cover in relation to the pipe and providing means for collecting liquid leaving the diffusion channel and for redistributing said thus collected liquid as a plurality of discrete substantially continuous streams.

5. A distributor according to claim 4 in which the liquid diffusing means comprises a mesh screen secured between said pipe and said cover to present its edges to the liquid flow so that liquid will be constrained to flow edgewise through the screen.

6. A distributor according to claim 4 in which said liquid diffusing means comprises a porous sintered metal.

7. A distributor according to claim 4 in which the said overwrap means comprises a helical wire positioned to provide about one-half inch between adjacent turns thereof for providing drip points spaced at intervals along said distributor.

8. A distributor according to claim 4 in which the said holes are spaced uniformly apart at a distance of about one inch.

9. A distributor according to claim 4 in which the holes are of 0.015 to 0.025 inch diameter.

10. A distributor according to claim 4 comprising a single inlet for passing said liquid into said pipe.

11. The combination of a rotary filter drum and a wash liquid distributor according to claim 4 in which the distributor is mounted radially outwards of and above the drum and parallel to the axis thereof whereby, during operation of the filter and distrubutor, wash liquid is distributed on cake on the drum in a plurality of discrete pools which may merge to form a continuous pool of wash liquid on the cake.