Note: Descriptions are shown in the official language in which they were submitted.
~ 37~7
The present invention relates -to Eluidi%ed bed
drying oE solid material containing vapourisable ma-terial
such as dryin~ oE solid material con-taining water or
removal of solvent from solid mater1al.
S Combustion o~ coal ~or power genera~ion is adversely
affected by high moisture content in the coal. This is
particularly so with brown coal which oLten'contains two
pounds water per-pound of dry coal'and thereEore presents
special problems in combustion. It is possible to incre~se
the efficiency of a boiler substantially by burning dry coal
instead of raw coal.
It is known -to dry brown coal prior to combustion using
hot combustion gases or air which has been' heated by steam or
hot gases and drawing or blowing the gas over or through the
brown coal in particulate form. However, -this method o~
drying offers no advantages compared to burning raw coal since
the steam driven off is admi~ed with gas. The gas/steam
mixture is simply vented to the atmosphere and its energy
content lost since'it is not' economical to attempt to recover
- 20 or use the energy. Also this method of drying is hazardous.
The present invention relates to a me~hod and apparatus
in which solid materials are dried and vapour resulting
from the drying is used for further drying purposes. The
drying system of the present invention is much less hazardous
than known drying systems.
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In our co-filed Canadian Patent Application No.
321,185 there is described in detail a method of drying
a solid material containing a vapourisable material whlch
comprises establishing a Eluidized bed containing the
s solid material in which the fluidizing medium is the
vapourisable material in vapour form and in which the
fluidized bed is heated indirectly, such tha~ vapouris-
able material is removed from the particulate material
for further use~
The invention of the above mentioned co-iled Patent
Application is described with particular reference to
the drying of brown coal in an electrical power station
environment where the brown coal provides the combustible
material which is burned to produce heat which is used to
convert water to steam and the steam used to drive turbines
; which generate electricity. However, that invention is of
general applicability where it is necessary to remove a
vapourisable material from a solid material. For example,
the method of that invention may be used for removing sol-
vent from particulate catalyst materials used in industrial
processes. Since the fluidizing medium is the vapourisable
material, in this case the solvent, the solvent is not con-
taminated with other fluidizing gases and can be readily
recycled for further use.
In accordance with one aspect of the present invention
there is provided a fluidized bed multiple effect drying
apparatus, comprising at least first and second fluidized
bed housingst means for feeding solid material containing
vapourisable material to each housing to form the bed,
means for removing solid material having a reduced va-
pourisable material content from each housing, means for
removing vapourisable material driven off from the solid
material from the housing, and means for indirectly
heating each fluidized bed, wherein means is provided to
enable the vapourisable material driven off from the first
fluidized bed to supply heat to the indirect heating means
of the second fluidized bed.
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In particular, the present invention relates to mul-
tiple effect drying or mechanical vapour recompres~ion.
According to another aspect of the invention there is
provided a fluidized bed multiple effect drying appara-
tus, comprising at least first and second fluidized bed
housings, means for feeding solid material containing
vapourisable material to each housing to form the bed,
means for removing solid material having a recluced va-
pourisable material content from each housing, means
for removing vapourisable material driven off from the
solid material from the housing, and means for indirectly
heating each fluidized bedt wherein means is provided to
enable the vapourisable material driven ofE from the first
fluidized bed to supply heat to the indirect heating means
of the second fluidized bed.
An example o such an apparatus is described in detail
in our co-filed Canadian Patent Application ~o. 321,185.
Other aspects of this invention are claimed in a
divisional application.
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The present inventlon is applicable to drying of
particulate material in which the particulate material
forms the Eluidized bed in the absence of any other solid
particles. It is also applicab~e to the ~ryLng o~ lump
material in which the fluidi~e(l bed contain~ another,
particulate material which constitutes ~he ~luidizing
material and the lump material is fed into the fluidized
bed. Drying of lump material is described in detail in
the co-filed complete specification based on our Canadian
Patent Application No. 321,185. The present invention
will be described hereinafter with reference to drying
of particulate material but it is to be understood that
it is equally applicable to drying of lump material.
In the present invention if there are two effects in
series, one pound of steam can in practice, allowing for
heat losses, evaporate at least 1.56 lb. water and if
there are three effects the evaporation can be at least
1.85 lb. water. The solids can flow co-current, counter-
current or cross-current with the steam.
The present invention will hereinafter be described
with particular reference to drying of coal but it
is equally applicable to drying of other particulate
materials.
In a co-current system coal is fed to a means for
introducing it to a fluidized bed, such as a lock hopper
where it is pressureized to the pressure of a first high
pressure fluidized bed. The coal in particulate form is
introduced into the first fluidized bed wherein it is
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2i37~7
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partially dried. The partially driecl coal is then
introduced to one or more lower pressure effect fluid-
ized beds wherein the pressure decreases in succession.
After exiting from the last ~luidized bed in the se~ies
it is sufficiently dried for use and is fed to storage
or boiler. The vapour generated in the first high
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pressure fluidized bed is -typically cleaned by cyclones and
Eed to the dryer tubes of -the next fluidized bed in the
series. This process is repeated for each o the fluidiæed
beds in the series.
Should coal particles accumula~e on the hea~incJ
surface, the condensate may be iltered and spra~e~ onto the
heating surface intermittently or continuously. The coal
and steam move in parallel paths in the same direction.
In a counter-current system coal is fed first to a
low pressure effect fluidized bed where it is partially dried
and then transferred to the next higher pressure fluidized
bed in the series. Steam is flrst admitted into the dryer
tubes of the high pressure effect. Vapour from the high
pressure effect is passed to the nex-t lower pressure effect
and so on. Thus, steam and coal move in parallel paths in
- opposlte directions.
- Where dried coal is put to different applications
e.g. combustlon for power generation and manufacture of liquid
fuels, a cross-current system may be employed. Such a system
may be desirable if partially-dried materials do not fluidize
well but materials fully, or almost fully, dried, do
fluidize well. In this case the steam as before proceeds
from high to low pressure effects. Coal is admitted
continuously to each effect. Thus, dry coal is available
from the high pressure effect at high temperature and from
lower pressure effects at correspondingly lower t~emperatures.
Preferably, the pressure employed in multiple effect
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systems ranges between 2500 p.s.i.a. and 0.75 p.s.i.a.
With brown coal volatiles are released at about ~70C
and this sets a practical ~pper limit to the temperature
of the coal in the hiyh pressure effect. Allowing for
some degree of s~perheat, which can on~y be maintained
when the coal contains less than ~0% moisture or, in
general, when the material is sufficiently dry to exert
less than the vapour pressure of water (or solvent) at the
temperature of the solid, and a temperature difference of
110C then only two effect operation is possible e.g. high
pressure effect at 240C and low pressure effect at 130C.
Alternatively, the lowest pressure effect must be under
vacuum operating at for example, 20C. If a lower tem-
perature difference of say lOODC were employed the high
pressure eEfects might be at 240C, intermediate pressure
at 140C and low pressure at 40C (under vacuum).
Multiple effect drying gives similar effects to those
which are obtainable by the methods discussed in the above
mentioned co~filed Canadian Patent application in which
the same steam is used for power generation and for drying.
An alternative method of achieving economical opera-
tion is to take the dirty vapour emitted by the dryer and
pass it to the heating tubes of another, conventiona:l dryer
which is fluidized by another gas such as air or flue gas.
If the former dryer is operating at atmospheric pressure
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then the dirty vapour leaving it will condense in the
( . - heating tubes of the latter dryer at 100C, appro~imately.
If the bed of the latter dryer is heated to 50C or 60~C
by the condensing vapour, such as steam, vapourisable
material will vapourise into the air or flue gas, ~Ihich
will leave saturated ~ith vapour a-t the bed temperature.
- In the general case, the alr or flue-gas will pass through
a cyclone or bag-filter system and possibly an electrosta~ic
precipitation. In the special case where coal drying,
e.g. brown coal drying, is undertaken prior to combustion
and air is passed into the fluidized bed, then after
passage through the cy~lones, the air may be fed to the boiler
Precautions would be necessary to ensure that the coal air
mixture did not fall in the ex~losive region for such
coal-air mixtures.~ In this case the water-vapour is a
d;luent which absorbs some of the heat of combustion of
the coal, with deleterious effect on the efficiency. The
second dryer is of conventional type and the other gas e.g.
` air or flue-gas entering may be at substantially higher
2~ temperature than the temperature obtaining in the bed.
This two-effect me~hod of operation, with
fluidization in one effect by another gas and in the o~her
by water vapour may be used in the power station in place
of the system whereby one steam-fluidized dryer is used
and the water vapour
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used to generate clean steam for the low pressure turhines
as is described in detail in our co-filed Canadian Applica-
tion No. 321,187.
Thus, in the power-station, stearn may he with~rawn
from a high pressure or intermediate pressu~e t~1rbine and
passed to the steam-fluidized dryer as be~o~e. Th~ dirty
vapour produced may pass to a dryer fLuic~ized with air
and after passing through cyclones to remove rnost oE the
entrained coal, the moist air passes to the boiler to
support combustion. Coal flows from the storaye bin into
the air-fluidized dryer in which about half the moisture
is removed. Then the "half"-dry coal is transferred to
the second bed which is fluidized by water-vapour and
the drying process carried to the desired final moisture
content.
Alternatively, coal is partially dried in the steam-
fluidized dryer and then finally dried in the gas-
fluidized dryer. Also, a further possibility is that
the two dryers may be operated in cross-flow so that each
is fed independently with coal and dried coa~ is removed
from each dryer. Instead of coal any particulate material
needing drying may be employed.
Thus, it is possible to have two effects operating at
the same pressure which is not possible when pure vapour
is used as the fluidizin~ medium which requires that each
effect operate at different pressure as described above.
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747
The dirty vapour produced from -the dryer in accordance
with the method of the present invention also lends itself
to mechanical vapour r~compression. The dir-ty vapour is
preferably first passed through a cyclone system to remove
solids which are preferably re-turned to the fluidi~ed bed.
The vapour fro'm the cyclone is then passed to a steam-generator
which produces clean steam by hea-ting the condensake from the
dryer tubes. The clean s-team will be saturated but may be
superheated before recompression to produce compressed
saturated vapour in an appropriate apparatus such as a
centrifugal compressor to a sufficiently high pressure for
an adequate heat transfer rate in the drier such as between
20 and 2000 p.s.i.a., preferably between 50 and 500 p.s.i.a.
The compressed steam after de-superheating e.g. by condensate
spraying, may be passed through the dryer tubes of the same
fluidized bed to effect fur-ther drying of raw coal.
The use of mechanical vapour recompression in which
the condensa*e from the dryer tubes is reheated and compressed
and used again can give in practice the same performance as
a two or three effect multiple dryer as discussed herein.
Further, carrier steam as discussed a-bove may be drawn
from the compressor at a low pressure such as less than
10 ~.s.i.a.,e.g. 5 p.s.i.a., above the fluidized bed pressure
- which will be preferably close to ,atmospheric, and fed to a
distributor at the bottom 'of the dryer. Alternatively, a
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separate fan may be employed to return product vapour to
the bed as carrier steam.
The present invention will now be described by way
of Example with reference to the accompanyiny drawings, in
which:-
Fig. 1 is a schematic flow diagram illu~traking aco-current multiple effect process in accordance with the
present invention;
Fig. 2 is a schematic flow diagram illustrating a
counter-current multiple effect process in accordance with
the present invention;
Fig. 3 is a schematic flow diagram illustrating a
cross-current multiple effect process in accordance with
the present invention;
Fig. 4 is a schematic flow diagram illustrating a
cross-current drying process for coal in accordance with
the present invention.
In Figure 1 there is shown a co-current triple-effect
dryer in accordance with thè present invention. The
apparatus comprises three driers 50, 52 and 54 in series.
Coal is fed from a hopper or other storage means 56 along the
schematic dotted line 58 through each dryer in turn starting
with the dryer 50. Steam is fed into the heating tubes of
the dryer 50 under high pressure and at a high temperature.
The steam transfers heat to the coal in dryer 50 and so
causes moisture to be driven off from the coal in the form of
dirty steam. The steam from the dryer tubes is converted
into a condensate which is subsequently processed for further
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use. The dirty steam is passed to a cyclone 60 and thcn -to
the dryer tubes of the dryer 52. Al-ternatively steam from
the cyclone 60 may pass to a clean steam generator the s-team
from which passes -to the dryer -tubes of the dr~er 5~. The
dryer 52 is operated at an intermedia-te pressure and
temperature. ~s with dryer 50 the s-team rom the dryer -tubes
is converted -to condensate and dirty steam is driven off
from the coal.
The dirty steam from dryer 52 passes -to a cyclone 62
and then to the dryer tubes of the dryer 54 or through a clean
steam generator the steam from which passes to the dryer tubes
of the dryer 54. The dryer 54 is operated at a low pressure
and temperature. Further, dirty steam is driven off from the
coal in dryer 54 and this dirty steam passes to a cyclone 64
i5 and is used for process heating or steam generation or is
passed to a condenser. The dried coal emerges from dryer 54
- in a relatively cool condition.
In Fig. 2 there is shown a counter-current triple
effect dryer ln accordance with the present invention. The
apparatus of Figure 2 operates in much the same manner as
that of Figure l except that the coal is first fed to low
pressure and temperature dryer 54, then -to the intermediate
dryer 52 and finally to the high pressure and temperature
-dryer 50. In this case the coal emerges in a relatively hot
condition.
In Figure 3 there is shown a cross-current triple
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effect dryer in accordance with the present invention. This
operates in a similar manner to the dryer of Figures 1 and 2
except that as coal is independently fed to each dryer and
removed dry the coal from each of the three dryers is at a
different temperature. Hot dry coal is obtained from dr~er
50, intermediate dry coal is obtained from dryer 52 and cool
dry coal is obtained from dryer 54.
In Figure 4 of the accompanying drawings, there is
shown, by way of example, a schematic flow diagram illustr-
ating a cross-current drying process for coal in which one
fluidized bed is fluidiæed by steam and the other fluidized
bed is fluidized by hot gas. The process illustrated in
Figure 4 can also be adapted for co-current and counter-
current systems.
In Figure 4 there is shown a first fluidized bed dryer
61 and a second fluidized bed dryer 63. Particulate coal is
fed from a bin 65 to the dryer 61 and from a bin 66 to the
dryer 63. Gas is fed along a line 63 through a preheater 70
in which it is heated to an appropriate temperature as is
known in the art, into a plenum 72 at the base of the dryer
61 and then into the fluidized bed of the dryer 61.
Steam obtained as dirty vapour from the dryer 63 is
fed along a line 74 into drying tubes 76 of the dryer 61.
The steam in the drying tubes 76 indirectly heats the
material in the fluidized bed to a temperature of about 50
or 60 C and
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itself condenses to leave the drying tubes 76 at a temperature
of about 100 C. The dirty condensate may -typically be fed to
a filter (not shown) through a line 77 to remove particula-te
material and then used as described above.
The heating of the fluidized bed in the dryer 61 causes
steam to vapourize into the hot gas which leaves the dryer
61 as saturated with steam at the bed temperature. r~he
saturated gas is then passed to a cyclone 78 from which re-
moved solids are returned to the fluidized bed of the dryer
61 and the gas is passed through a filter such as bag-filter
or electrostatic precipitator 80. The gas from the filter,
if air, may proceed to a boiler furnace through a line 82.
Dry coal is withdrawn from the dryer 61 through a line
84.
With the dryer 63, saturated steam, for example from
a boiler or turbine, is fed into drying tubes 86 through a
line 87. The steam in the drying tubes 86 indirectly heats
the material in the fluidized bed to a temperature of about
llO C and itself condenses. The condensate from the drying
tubes 86 is fed through a line 88 to a boiler. The fluidizing
medium in the dryer 63 is steam which is given off from the
fluidized bed as dirty steam relatively uncontaminated by air
or other gases. The dirty steam is passed to a cyclone 90
wherein particulate matter is removed and returned to the
dryer 63. The steam from the cyclone 90 is fed partially
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along the line 74 to the dryer 61 as described above and
partially along a line 92 through a pump means 94 into
a plenum 9~ at the base oE the dryer 63.
The steam fed into the plenum 96 passes into the
fluidized bed and serves as carrier vapour as descibed
hereinabove.
Dry coal is removed from the dryer 63 through a line
98.
In the system shown in Figure 4 both systems operate
at substantially the same pressure.
The multiple effect drying system of the present
invention has particular application to the hydrogenation
of coal in which it is important to have an economical
drying method.
Modifications and variations such as would be apparent
to a skilled addressee in the art oE drying particulate
material are deemed within the scope of the present in-
vention. For example, the multiple effect drying and
mechanical vapour recompression systems and the two-stage
system with one dryer fed with non-condensible gas of
the present invention may be used in connection with an
electrical power generation system as described in detail
in our co-filed Canadian Patent Application No. 321,187
entitled "Power Generation System".
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