Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
The present invention relates to a process and
an apparatus in the separation of solid catalyst par
ticles suspended in liquid.
It is customary, in process technology, to carry
out reactions between gas and liquid or one or more
substances dissolved in liquid with the aia of cata
lytically active solid substances present in liquids.
Examples of such reactions are hydrogenation, oxidation
and halogenation. The catalyst may be in the form of
a fixed bed through which the liquid passes, or a sus-
pension of particles which, by the application OL energy,
are held freely suspended in the liquid. The suspension
catalyst offers certain advantages, such as being suc-
cessively renewable in continuous processes In several
cases, the dispersed catalyst retains its activity for
a considerable period of time, and if the reaction lS
to proceed continuously, the catalyst must therefore
be retained in or constantly recycled to the reaction
chamber through which the liquid passes. This can be
accomplished by withdrawing the finally reacted liquid
through a filter retaining the catalyst particles.
In general, the instantaneous filtrate flow through
a filter is directly proportional to the filter surface
and the pressure drop across said surface and inversely
proportional to the viscosity of the liquid, the total
amount of deposit on the filter surface, and a constant
whlch depends on the permeability of the deposit.
It is already known that a filter surface can be
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instantaneously freed frorn deposits by so-called back-
washing, implying that liquid is pressed through the
filter in a direction opposite to that of the normal
filtrate flow. U.S. patent specification 2,990,238 teach-
es backwashing with the aid of a piston or diaphragm
pump which in one stroke direction exhausts filtrate
and in the other stroke direction presses a part of
the filtrate back through the filter.
As will appear from Fig. 1 of the said U.S. patent
specification~ only half the available time is utilised
for the wi*hdrawal of filtrate, regardless of the pump
stroke frequency which is given as 1-500 strokes per
minuteO Furthermore, it appears that the backflow should
amount to 20-70~ of the amount of filtrate exhausted,
and this results in a further corresponding reduction
of the product flow. According to this procedure, the
backwashing flow cannot be greater than the filtrate
flow.
German patent speciflcation 1,542~0B9 shows a filter
arrangement for use in continuous hydrogenationO The
filtrate is led out through a stationary centrifugal
pump. When the pump is started, filtrate is pressed
back through the filter. Hydrogenation is carried out
by the circulation of liquid and hydrogen gas dlspersed
in the liquid, the flow rate according Example 1 being
0O7 2.8 m/s~ The filter is placed laterally of the cir-
culation flow, for which reason the flow rate during
filtration will approach zero at the filter top. Accord-
,
~2~
ing to the description, backwashing can be effected
for a few seconds. The concentration of solid particles
is given as 1-5 g/l in the Examples.
British patent specification 959,583 discloses
the hydrogenation of a liquid consisting of alkyl anthra-
quinone dissolved in organic solvents. This hydrogenation
is a partial process in the well-known anthraquinone
process for the production of hydrogen peroxide. The
hydrogenation catalyst suspended in the solution is
retained on a filter which is periodically backwashed
with the aid of a pump. No further information about
the backwashing and the catalyst is given.
German patent specification 1,272,292 exemplifies
the same hydrogenation process. The filter medium employed
is a porous solid carbon material. The anthraquinone
solution contains as catalyst 0.07% by weight palladium
black. The particle size of the catalyst is 0.01-1 ~m.
The filtra~e flow is 0.46 m3 per hour and m2 of filter
surface. According to the description, the carbon fil-
ter is to be backwashed for 3;10 s at intervals of
20-30 min.
Backwashing can be carried out by means of; for
example, a pump directly connected to a special back-
washing conduit or, as is shown in German patent spe-
cification 1,542,098, inserted in the filtrate conduit.
In some processes, such as the hydrogenation comprised
.
by the anthraquinone process for the production of hydro-
gen peroxide t the amount of filtrate per unit of time
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may be considerable~ and this means that a directly
connected backwashing pump must then also have a large
instantaneous capacity, es~ecially when the backwashing
flow is to be greater than the filtrate flow This tech-
nique may complicate the automation of the backwashing
procedure and constitutes an obstacle to the optimisa-
tion of the backwashing in each particular case.
For the hydrogenation comprised by the anthraquinone
process) use is made, in addition to palladium black,
also of Raney nickel or, for example, palladium provided
on a so-called carrier, usually a ceramic material.
These suspension ca~alysts are used in concentrations
higher than 5 g per litre of liquid, and the average
particle size is greater than 1 ~m.
The use of such catalysts has made the discharge
of product liquid through filters so difficult that
the catalyst separation in some cases has been carried
out by means of hydrocyclones and/or centrifuges.
The present invention provides a process in the
separation of solid particles suspended in liquid, such
as Raney nickel, said particles being catalytically
active in continuous chemical processes, such as the
hydrogenation process comprised by the anthraquinone
process in the production of hydrogen peroxide, more
than 75% of said particles having a particle size greater
than 1 ~m, and said particles being present in a con-
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centration which~ on an averàge, is maintained above
5 g per litre of liquid, the liquid containing the reac-
.. . .
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tion product being caused, after passage of a reaction
chamber, to pass through one or more filters, the solid
particles thus retained on the filter surfaces being
released by periodic backwashing and then resuspended
in the reaction chamber. The process is characterised
in that backwashing is carried out with a liquid which,
before a backwashing run is started, is placed under
a pressure higher than the pressure in the reaction
chamber and to which has been imparted~ or which has
been placed under the action of, a stored amount of
energy sufficient to start and maintain a liquid flow
through the filter or filters during backwashingO
The present invention also provides an apparatus
for separating solid catalyst particles suspended in
liquid, said apparatus comprising a reaction vessel
for reaction between one or more substances dissolved
in the liquid, in the presence of the suspended solid
catalyst particles and one or more filters disposed
in the outlet conduit of the reaction vessel for re-
taining the solid catalyst particles within the reac-
tion vessel. The apparatus is characterised in that
there is connected to the outlet conduit a container
for storing backwashing liquid, said container being
under a pressure higher than the pressure in the reac-
tion vessel and hav1ng a stored amount of energy suf
ficient for backwashing said filter or filters.
The present invention makes it possible to obtain,
under long-time continuous conditions, an effective
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specific amount of filtrate which is greater than 0,5 m3
per hour and m2 of filter surface, also when the cata-
lyst concentration is greater than 5 g per litre of
liquid and/or the deposit on the filter has low permeabi-
lity.
It has been found that an especially advantageous
backwashing with minimum application of energy is ob-
tainable if the backwashing is carried out with a liquid
which, when a backwashing run is started, is under the
action of or in itself contains a stored amount of energy
sufficient for carrying the backwashing into effect.
The energy in question may be, for example, a positional
energy, i.e. the backwashing liquid is stored in a con-
tainer in such an amount and is placed at such a level
that the hydrostatic pressure of the liquid during the
entire backwashing run is higher than the pressure in
the vessel containing the filter, It is preferrPd, how-
ever, to use a so-called hydrophor or liquid elevator.
The deposit on the filter constitutes not only
an obstruction; it also serves as a filter intercept
ing those particles which otherwise could have passed
through the pores in the solid filter. During each back-
washing, the filter surface is exposed for such penetra-
tionO In view hereof, and in order to obtain maximum
discharge of the product flow, lt has proved to be most
advantageous to effect backwashing at intervals from
1/3 minute to twenty minutes, depending upon the con
centration of solid particles in the llquid and the
permeability of the depositO
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36~3
Backwashing is intended not only to wash off the
outer deposit on the filter surface, but also, if pos-
sible, to wash back any particles that may have pene-
trated into the pore system of the filter medium.
To this endt the backwashing according to the in-
vention must be carried out with an amount of liquid
which is greater than the one enclosed in the filter
medium, preferably 2~10 times greater than this amount.
Furthermore, the specific backwashing flow must be great-
er than the specific filtrate flow during the first
twenty seconds after backwashing. It has also proved
to be advantageous to impart to the filter surfaces
facing the reaction chamber a mo~ement adjacent said
filter surfaces which, on an average, is greater than
Ool m/s.
Preferably, more than 75~ of the catalyst particles
have a particle size greater than 1 ~m.
The invention will now be illustrated with reference
to the accompanying drawings in which
Fig. 1 illustrates an apparatus according to the
invention;
Figs. 2 and 3 are diagrams showing the instantaneous
filtrate flow during backwashing as a function of the
time~
Fig. 1 illustrates as an example an apparatus for
reaction between gas and liquid by means of a suspended
catalyst. The reaction is carried out under an exces~
.
pressure in a reaction vessel 1. The liquid is supplied
continuously through a conduit 2, and the gas continuous-
ly through a conduit 3. The catalyst is held suspended
in the liquid by means of an agitator 4. The liquid
reaction product is pressed out through a filter 5 and
a conduit 6, with interruptions only for backwashingO
A backwashing run is started~ for instance, by
rotating a threè-way valve 8 through 90 and is stopped
by resetting the valve to initial position. During the
backwashing run, liquid is pressed from a container
9 which is under a gas pressure higher than the one
in the vessel 1, back through the filter 5. During back-
washing, the pressure in the container 9 is decreased.
The decrease in pressure provides, in some suitable
manner~ an impulse such that a pump 10 begins to press
liquid into the vessel 1 until the desired value of
the pressure has been reestablished preparatory to the
next backwashing run.
Gas may be supplied to the container 9 through
a conduit 11. The gas in the container 9 should be inert
relative to the gas and the liquid in th~ vessel 1.
In~ for instance, hydrogenation processes use is made
preferably of nitrogen or carbon dioxide gas.
The invention will now be explained in greater
detail with reference to the following non-restrictive
Examples.
- EXAM
As a step of the anthraquinone process ~or the
production of hydrogen peroxide, anthraquinone derivative
, .
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was hydrogenated with hydrogen to the corresponding
anthrahydroquinone derivative in an apparatus accord-
ing to Fig. 1. 1,000 litres of working solution/min.
were supplied continuously to the vessel 1. On an average,
approximately the same amount of filtrate was discharged
through the filter 5~ Backwashing by means of the liquid
elevator 9 was carried out for 1 second during each
minute with an amount of liquid of about 30 litres/back-
washing. In this procedurep the pump 10 has a capacity
of at least 30 litres/min
If there had been no pressure vessel 9, and if
the pump had been directly connected to the conduit
7, this would have necessitated a pump capacity which
is 60 times higher, and furthermore the power required
for operating the pump motor must be 60 times higher.
This shows the great saving of energy obtainable with
the present invention
EXAMPLE 2 ifor comparison)
-
Fig. 1 illustrates the filtration run upon hydroge-
nation of anthraquinone derivatives dissolved in organic
solvents. The solution contained 110 g/l of suspended
Raney nickel having a particle size of from about 1 ~m
up to 0.5 mm, more than 75% of said particles having
a particle size greater than 1 ~m. The filter medium
consisted of sintered particles of acidproof steel.
The material thickness was 2 mm, and the porosity amount-
ed to about 40~ ~aximum pore width was 8 ~m.
Before the filtration run, the pore system of the
, .
36~8
filter medium had been chemically cleaned. Through appli-
cation of mechanical energy, a turbulent flow rate which,
on an average, was greater than 0.1 m/s, was imparted
to the solution at the filter surface. Backwashing was
carried out for 6 seconds, and the filtrate flow was
then shut down for altogether 12 seconds. Backwashing
was carried out at intervals of lO minutes.
After deduction of the hydrogenated solution back-
washed through the filter, the average filtrate flow
during each lO min. period was about 1.3 m3 per m~ of
filter surface and hourD After continuous operation
for ten days, the effective filter flow was less than
l.0 m3/m2. The hydrogenation formed part of the anthra-
quinone process for the production of hydrogen peroxide.
Fig. 2 illustrates the filtration run under the
same conditions as in Example 2. Backwashing was car-
ried out for 2 seconds during each minute. The average
effective ~iltrate flow was 2.25 m3/m2/hourn After con-
tinuous operation~for 60 days the effective filtrate
q
~ flow was about 2.1 m~/m2/h~urO
.
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11
