Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
The invention relates to a process for the removal of
water from a mixture substantially made up of gasification coal
and water. By gasification coal is to be understood here the
coal obtained in the gasification processes known in the art of
the preparation of gas mixtures containing carbon monoxide and
hydrogen by incomplete combustion of heavy hydrocarbons with the
aid of oxygen-containing gases.
Known gasification processes are among others the
Shell gasification process and the Texaco gasification process.
These processes are described~ for instance, in Hydrocarbon
Processing~ Vol. 46, No. 11 (The 1967 Petrochemical Handbook
Issue~, November, 1967, p. 227, and in Industrial and Engineering
Chemistry, Vol. 48, No. 7, pp. 1118-1122, and in the B~itish
Patent Specifications 734,475 and 780,120 and the United States
Patent Specification 2,914,418.
The gasification coal prepared by these known processes
has a surface area, determined by the BET-method, of 600-1500
m /g., a micropore volume (N2-method) of 0.8-4 ml./g.~ a
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macropore volume (Hg-porosimeter) of 1.5-5 ml./g. and an oil
adsorption of 2-6 ml./g.
In the gasifica-tion processes, the gasification coal
is usually separated from the resulting gas mixture by applying
a scrubbing treatment with the aid of, for instance, a water
curtain, generally resulting in the formation of an aqueous
suspension containing 0.3 - 4% by weight of gasification coal.
In practice, the gasification coal is subsequently separated from
the water, on the one hand, to make it possible for the water
to be recirculated or discharged, and on the other hand, to
recover the coal, which has valuable properties and may be
applied for instance as absorption coal. In the separation of
the gasification coal from the water, use is generally made of
a water-immiscible solvent, for instance, toluene or a mineral -~
oil.
After the water has been separated, the mixture of the
solvent and the carbon black agglomerates formed therein is
subsequently split up into the two components by evaporation of
the solvent. Such processes are described in, for instance,
the British Patent Specification 741,135. Disadvantages to
these processes are the high flammability and risk of
explosion of the solvent, the usual losses of solvent and the
fact that an effective washing of the resulting coal agglomerates
cannot be integrated into the process in a simple way. Moreover,
these processes call for a relatively high capital outlay and
operating costs.
In practice, however, for lack of a better process,
the utmost is done to limit the hazards to the present method
and the losses of solvent by taking the greatest possible `
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precautions. In the literature, it has been proposed to thicken
a coal suspension mechanically, for instance, by sedimentation,
centrifuging or filtration, resulting in pastes containing only
6 - 10% by weight of coal, which can only be realized, however,
by using relatively large amounts of additives (see, e.g., the
Netherlands Patent Application 7,203,046). After granulation,
this coal paste is further dried by evaporation of water, for
instance, in a hot air stream. A disadvantage to this method,
however, is that it leads to high energy consumption per
kilogram of coal. Another drawback is that it is very difficult
to thicken an aqueous suspension of gasification coal to a
satisfactory coal concentration by sedimentation, centrifuging
or filtration under pressure or not; this particularly applies in
the case of suspensions of gasification coal having a surface
area, determined by the BET-method, of at least 600 m /g.
DESCRIPTION OF THE INVENTION
The present invention provides a method of mechanically
thickening an aqueous coal suspension to a coal concentration of,
for instance, 12 to 25% by weight, yielding a coal concentraie
that can in a usual way be worked up into some shaped product.
Partly due to the fact the coal suspension can be thickened to
a concentration which is 2 to 4 times as high as that obtained
before, there will be a considerable saving in energy in that
far less water need be evaporated.
The process according to the present invention is ;
characterized in that the mixture is expressed dynamically with
the aid of a band filter press and per cycle the filter bands, 7
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between which there is the coal-water mixture, several times
change direction while making an angle of at least 90, at least
one of the bands being permeable to water. Preferably, the bands
change direction 3 to 50 times, more particularly 5 to 20 times.
By dynamic expression is to be understood here a
process in which the coal particles are so shifted that they will
take up essentially different positions relative to one another
under the influence of differently directed shearing forces.
The filter press, which is applied for several other
purposes, is not found to be suitable to be used in the present
process. A screw press, however, for instance, a double-screw
press, is not found satisfactory in practice either, because,
although for a short time, a satisfactory thickening effect is
obtained, the fine-meshed wall sieve will eventually get clogged.
Use of such a sieve, however, is required because otherwise
insufficient pressure is built up to thicken the water-rich
coal-water mixture to the desired coal concentration.
Before the aqueous coal suspension is concentrated
according to the process of the present invention, it may be
subjected to a pre-treatment, but this is generally not necessary.
; A possible pre-treatment is a flocculation followed by decanta-
; tion and/or filtration with the use of, for instance, a vacuum
filter, a separator, or a decanter. If desired, the pre-treatment
and the process according to the invention may be carried out in :
one and the same apparatus.
The coal concentration of the suspensions to be
thickened according to the process of the invention is generally
within ghe range of from 0.2 to lO~ by weight, and preferably
within the range of from 0.5 to 5% by weight. At these concentra-
tions, the suspensions to be used are liquid or in the form of
a paste.
In addition to the description given above, the band
press is further described in the November, 1974, Issue of
"Process Engineering", at Page 70, in a review entitled "Belt
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Filters Apply the Pressure to Sludge". In the belt press as
employed in the present invention, there are horizontal and
vertical dewatering zones with shearing over offset rollers .
The essential features of the present invention are characterized
in that a mixture of gasification coal and water is passed
into a pressure zone wherein the mixture is compressed to expel
water. ~hile in the pressure zone, the applied pressure is
changed by reversing the dîrection of flow of the mixture into
the pressure zone thereby~disorientating individual carbon
particles in the gasification coal. Thereafter, the flow of
the gasification coal/water mixture into the pressure zone
is resumed to further compact the coal to a level greater
than was achieved during the preceding compaction step. The
sequence of reversing and resuming flow may be repeated a
~ large number of times as the mixture is passed through the
- pressure zone.
~ The following description of the figure describes an
;~ embodiment of the invention~
With reference to the Figure, there is depicted in
- 20 schematic form a press (called a "band" or "belt" press~ compris- ~-
ing drive rollers 1 and 2 supporting and actuating belt 3.
Rollers 1 and 2 are coordinated to drive belt 3 in a given
direction and are also reversible. Similarly, reversible rollers
6 and 7 support belt 8. Belts 3 and 8 converge to form a nip
(indicated generally by arrow 10). In operation, a paste-like
aqueous dispersion of gasification coal 15 is fed by plunger 12
along table 14 into nip 10 where the coal is compacted to
; squeeze out water which is absorbed into belts 8 and 13. These
belts pass into driers 20 and 21 where water is evaporated from
the belts and thence back into the nip 10 to extract more water
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i.e. rollers not in physical contact with one another
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from the gasification coal. After passing through nip 10~ the
gasification coal passes onto collection table 30 which is
fitted with a doctor blade 31 to skin the coal from the belt
surface 8. Coal is removed from belt 3 by doctor blade 32 and
; falls onto the surface of table 30.
~uring operation of the press, the normal forward
motion of belts 3 and 8, together with the action of piston 12,
forces the gasification coal into nip 10 to be comprr~ssed so
that water is removed. Unfortunately, the coal particles are not
easily compressed to their closest orr,imost densely packed state.
- It has been found that a much less voluminous packing (with
` correspondingly greater loss of water) can be achieved by
periodically reversing the direction of belts 3 and 8. It is
believed that this reversal tends to disorient the particles.
Thus, when the belts are again reversed to apply pressure, the
coal can~be compressed further than was possible before the
original reversal of the belts. Advantageously~ the pressure is
removed or reduced by reversing the belts from 3 to S0 times per
cycle. It is understood, for example, that a cycle is defined by
the passage of any point on belt 3 from point A through nip 10 and
back again. Therefore, while the predominant motion of belts 8
and 3 would be in the direction of arrows F, the belt direction
would be reversed contra to the arrows from 3 to 50 (preferably,
5 to 20) times per cycle. For most efficient operation, belts
3 and 8 are synchronized so they both move simultaneously in
the same direction. It can be seen that belts 3 and 8 make an
angle of 90 with the direction ~f flow of the gasification coal.
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One advantage of the process according to the invenbion
is that the coal mass issuing from the dynamic expression
apparatus is so consistent that it can be easily granulated or
otherwise formed. Another advantage is that the granules obtained
have sufficient stability of shape to withstand mechanical
transport and can be dried in a stationary or moving oven without
the granules deforming or lumping during these treatments. The
resulting granules are moreover of sufficient stability of shape
to further serve as shaped coal without requir~ng any further
treatment.
A further advantage is that the resulting coal
concentrate cah, if desired, at both before, during, and after
dynamic expression, be purified in a simple manner. Finally, ~;
the process according to the invention offers the advantage that
no additives such as binders or flocculants need be added to the
water-coal mass.
EXAMPLE
To a band filter press (Bellmer-Winkle press) there
- was fed a 2% by weight aqueous suspension of gasification coal
at a rate of 600 liters per hour. Per cycle, the filter bands ~
charged with the suspension change direction 4 times while making ~ -
an angle of 180 and two times an angle of 145 . The gasifica-
tion coal was obtained by partial oxidation of bunker C oil and
had a surface area, determined by the LET-method, of 950 m2/g.,
a micropore volume (N2-method) of 1.8 ml./g., a macropore
volume (Hg-porosimeter) of 3.2 ml./g., and an oil absorption of
3.2 ml./g. From the press, there was discharged per hour 75 kg.
of a water-coal mass with a coal concentration of 16% by weight
and about 525 liters of water which contained hardly any coal.
The gasification coal obtained after complete drying of the
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resulting granules had a porosity and a surface area which did
not differ from those of the coal in the starting suspension.
; The press can be in continuous operation for at least 300 days
without any difficulties. For comparison, the 2% by weigh-t
aqueous suspension was fed to a 4000 g-continuous disk separator,
as a result of which the coal concentration of the suspension
was merely increased to 3.3% by weight. Also, for comparison, `
the above-mentioned suspension was fed to a double-screw press
(manufactured by Stord-Bartz, Bergen, Norway). Within one hour
after the feeding had been started during which a water-coal
mixture with a coal concentration of 12% by weight was obtained,
the screw press was clogged and had to be taken out of operation.
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