Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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~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
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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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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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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
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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
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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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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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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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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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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
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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
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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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