Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION.
It is well known that the productivity of an oil
or gas well may often be increased by a procedure
jwhich involves crea~ing a fracture in the subterranean
5 Iformations surrounding a well and propping the fracture
opening by filling it with granular material called
propping agent. Methods of this type are disclosed in
US-patent specifications Nos. 3 701 383 and 4 068 718.
¦ A survery of propping agents and their manufacture
is given in the specification to German Offenlegungsschrift
29 21 336.
A granular material must fulfil several condi-
Itions to be suitable for use as a propping agent. The
15 ¦material must have hiyh strength to avoid crushing of
the particles when exposed to high pressure during
their application~ The shape of the individual particle
should depart as little as possible from the spherical
one and the particle size distribution should be within
defined, relative narrow limits to insure sufficien~
gas and oil permeability of fractures propped with the
propping agent. Moreover, the particles should be able
to resist the corrosive conditions to which they may
be exposed at their application.
~5 The material regarded as most suitable for ful-
filling these conditions is sintered bauxite pellets.
Several methods have been proposed ~ox producing
sintered bauxlte paxticles. The process which has hither-
'to found widest commercial success is the one described
in the above German Offenlegungsschrift 29 21 336.
According to said methods bauxite spheres arefirst prepared by agglomeration of a mixture of bauxite,
temporary binder and water in an intensive mixer to
produce spheres called green pellets, which are after-
wards sintered by heating. In the embodiment examplesof said application the products have a typical density
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of 3.7 g/cm3, while the crushing loss measured by the
method described below was 8,16~ and 6,~, resp.
It is however, a drawback of said process that
the granulation, which is carried out in an intensive
mixer, can only be perEormed batch-~ise. Moreover, said
prior art process usually requir~s a preliminary drying
of the starting material.
SUMMARY 0~ THE INVENTION:
It is, however, generally recognized that us~a~ly
a continuous process is to be preferred to a batch-
wise process when a large-scale industrial production
is concerned.
It is therefore an object of the invention to
provide a process for the production of sintered bauxite
spheres comprising steps all of which may be performed
continuously.
Since any crushing of the bauxite spheres by
~:~ their use as fracture propping agents involves an im-
paired permeability of the propped fracture it is
desired to obtain a strength of spheres corresponding
to an crushing loss even lower that the results obtained
according to said German Offenlegungsschrift 29 21 336.
Consequently it is an other object o~ the inven-
tion to provide a process resulting in bauxite spheres
havlng a crushing 1QSS measured by the below defined
method of les~ that S%.
In the above prior art process using an intensive
mixer the possibilities are rather limited for control-
ling the granulometry of the product, e.g. because a
certain minimum mixing intensity will always be necessa-
ry to secure homogeneity of the individual particles.
This means that a product having a very narrow particle
size distribution, as is often required for propping
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agents, can only be prepared by simultaneous product.ion
of a substantial amount of partlcles being too small or
: too large, which necessitates extensivé sieving opera-
tions and impairs total process economy.
Therefore it is a further object of the invention
to provide a process which may easily be adapted to
produce a major fraction of particles complying to
varying requirements as to particle size distribution.
I The above objects are according to the invention
achieved by a process for the production of sintered
bauxite spheres having a particle size range from 0O4
to 2.5 mm, or a more narrow particle size range within
said limits,suitable for use as fracture propping agent
in oil wells, which process comprises the steps of
(a) preparing an aqueous feed suspension comprising
bauxite and a binder,
(b) continuously atomizing said feed suspension into a
layer of alxeady partly dried bauxite particles
fluidized in a stream of drying air,
(c) continuously recovering particles from said layer
(d) continuously separating said recovered particles in
oversize, unde.rsize and product fractions, making
allowance for shrinkage in the subsequent sintering
operation,
(e) continuously recycling material selected :Erom the
group cons:Lsting of undersize fractions, xelat.tve
f:ine product fractions, ground product fractions
and ground oversize fractions, to the layer o
fluidized particles at a site at substantial
distance, measured along the flow path of the par-
ticles,from the site where said recovering of par-
ticles takes place, and
(f) drying and sintering the non-recycled product
fractions by heating them at a temperature between
1200 and 1650C.
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A method involving steps in principle correspon-
ding to the above steps (b)-(e) is called ~luidized
spray granulation and has been suggested ~or the granu-
lation of various inorganic and organic products. Spray
granulations has, howe~er, not been suggested for the
manufacture of green proppant pellets and the suitabili-
ty of the method for this specific purpose could in no
way be predicted. In ~act it is rather surprising that
a perfect spherical shape of the pellets ca~ be obtained,
considering the very rapid evaporations o~ the atomized
suspension in the process.
The crushing strength of the resulting sintered
particles is to a substantial extent dependent on the
homogenity of the green proppant pellets, and it is
; 15 also rather surprising that a spray granulation process
is able to produce pellets which are supperior also
in that respect.
In the present specification and in the accompany-
ing claims "bauxite" is used in the widest sense of the
word comprising also very low grade materials.
All steps of the process of the invention may be
carried out in a continuous manner, which makes -the
process especially attractive for large scale proppant
manuEacture. It is a further advant~ge of khe process
that -the prellminaxy drying necessary in prior art
methods is disposed oE.
By using the steps (a)-(e) it :Ls possible to ob-
tain pellets of spherical shape and of sufEicient
strength for handling prior to and during the final
drying and sintering process. After sintering these
pellets form propping agents of higher strength than
described in the prior art.
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DESCRIPTION OF THE DRAWING.
The invention is further elucidated with reference
to the drawing, which is a schematic flow sheet illu-
strating an embodiment of the process.
S DESCRIPTION OF THE INVENTION.
On the drawing a feed tank is designatPd 1. In
this tank an aqueous feed suspension comprising bauxite
and a binder is prepared. Preferably the feed contains
40-60, more preferably approximately 50 t % by weight
bauxite (as defined abov'e~ and preferably 0.25-5, more
preferably 0.5-2.5, % by weight binder. The bauxite
should preferably have a particle size below 20 micron
which îs conveniently achieved by a wet grinding process
which is less energy consuming than dry milling
prescribed in connection with some of the prior art
processes.
The preferred binders are polyvinyl aceta~e, polyvinyl
alcohol, methylcellulose, dextrin and molasses.
The function of the binder is to provide green
20 1 strenyth to the pellets until the sinterlng thereo~.
During the sintering most of the binders comming lnto
consideration will decompose.This means ~hat ~ relakively
h:Lyh amount Oe binder will impair the str~n~th Oe the
e inal sintexed product, for which reason blnders are
preEerred ~hich exhibi-t ~ sueficient temporary binding
ahility even when used in small amounts.
Also further auxiliary agents may be added to the
feed, such as dispersing agents, e.g~ ammonium citrateO
From the tank 1 the feed is led to a pump 2
feeding atomizing nozzles 3 arranged in a fluidized
bed unit 4.
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:~ Between the feed tank 1 and the nozzles 3 may
be inserted a grinding mill and/or a sieve tnot shown),
to prevent that too coarse particles reach the nozzles
and the fluid bed.
The atomizing nozzles 3 are pressure nozzles o~
conventional design~ or two-fluid nozzles. The design
of such nozzles is well known e.g. from K. Masters:
''Spray Drying Handbook", John Wiley and Sons, New York
` ~1979).
The fluid bed unit 4 is of conventional desig~
Ias described in e.g. US patent specification 3 533 829
and in British patent specification 1 401 303
In the illustrated~em~odiment a fluidized par-
ticle layer 5 is supported by a perforated plate 6
through which hot fluidizing gas is flowing. Said hot
gas is introduced in the bottom part of the fluid bed
unit by means of a fan 7 and an air heater 8.
The distance from the atomizing nozzles 3 to
the ~erforated plate 6 is adjustable and the nozzles
20 1 are pr~ferably possitioned a rather short distance
¦above the surface of the fluidized particle layer 5.
The exact position of the nozzles will in each indivi-
ldual case be fixed with due regard to the fact that too
! long distance from the nozzles to the surface of the
fluidized layer causesundesired dus-t formation, because
the atomized feed droplets will be dried to -too high
an extent, before they reach the fluidized layer, whi].e
a too short distanae on the other hand results in for-
mation of irregular and too coarse particles. Therefore,
the position of the nozzles is adjusted on the basis
of analyses of powder taken out from the fluid bed
unit.
The v~locity of the fluidizing and drying air
passing the layer 5 is preferably 0 T 9-1.5 m/sec.
and the height of the fluidized particle layer will ty-
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pically be 20-~0 cm.
The temperature oE the drying- and 1uldiziny air
when introduced at -the botkom part of the fluid bed unit
- 4 will preferably be 250-650 C, more preferably 400-
600C.
When leaving the fluid bed unit the tempera-ture
of said air is preferably below 100C, more preerably
- approximately 70C.
- From the fluid bed unit the ai~ entraining dust
consisting primarily of fine bauxite particles are led
to a collector means 9, which may for instance be an
electrostatic precipitator, a cyclone, a bag filter or
a wet scrubber or a combination thereof.
Particles recovered in the collector means 9
may be recycled to the feed tank 1 and/or to the
fluidized bed unit 4. It has turned out that the fine
paxticles collected in 9 are very suitable for being
recycled as seed particles to the fluidized bed due
to their uniform spheroidal shape, and in this respect
they are supperior to other seed particles obtained by
grinding as explained below.
It is essential tha-t -the fluid bed unit 4 is
designed to give a long and uni~orm residence time or
the particles to obtain a desired particle ~.Lze distri~
25 jbu-tion and the des.ired spherical shape o~ the procluct.
Therefore, the ~low of particles ln the fluidizcd layer
shoul.d be o~ the type conventionally termecl plug Elow,
which is a ~low pattern wherein very li-ttle back mixiny
takes place. Thereby an equal treatment of all particles
is secured.
In a fluidized particle layer plug flow of the
particles may be obtained by various measures. In the
embodiment shown on the drawing the desired flow pattern
is obtained by introducing powder particles serving as
seeds or nuclei through a powder inlet 10 in one end of
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the fluid bed uni-t 4 and removing particles from the
fluidized layer 5 through an outlet 11 situated at
the opposite end o~ the ~luid bed unit. Alternatively
- plug flow may be obtained by using guide walls in ~he
fluid bed as is well known in the art.
The seed or nuclei particles introduced through
powder inlet 10 consist of rec~cled material as will
be explained a~ove and further elucidated below.
I Alternatively to the illustrated embodiment the
10 ~fluid bed unit may comprise two or several compartments
in which different conditions prevail as to fluidizing
air velocity, temperature and slurry feeding conditions
~Such fluid bed units having more than one compartment
are well known in the art, and may e.g. have a circular
perforated plate and radian partitions preventing back
mixing.
Through the powder outle-t 11 is withdrawn a pow-
der having a moisture content of 1-5~ which powder via
a rotary valve 12 is conducted to a sieving unit 13
wherein it is separated into three or more fractions,
viz. an oversize fraction, one or more product fractions
- (in the embodiment shown: two fractions) and an under-
- size fraction.
The oversize ;fraction is conductecl to a grind:Lny
unit comprising a mill 14 and a ~ieve 15 which may
possibly be comhi.ned. Oversize makerlal are recycled
from the sieve 15 -to the miLI. 14 and fractions
having preferably a slze of app. 0.5 mm are, in the
embodiment shown, led to the powder inlet 10 of the
fluid bed unit together with the fine fraction. In case
the quantity of material of these two fractions, together
with material collected in 9, is not sufficient to form
seed or nuclei material for the fluid bed, a part of the
product fraction or of one of the produc~ ~ractions may
be added thereto as indicated by the dotted line in the
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lower part of the drawing. A part o or the total
amount of recycled product fraction may be ground before
being introduced to the fluid bed, as indicated on the
drawing. Qn the other hand, if the amount of material
in the oversize fraction together with the undersize
fraction is higher than what is required to supplement
seed or nuclei material, a part thereof may be added
to the feed tank 1, as illustrated by the dotted
line in the left upperpart of the drawing.
10 j Non-recycled product fraction or fractions are
led to a drying oven 16 wherein residual moisture and
organic additives are evaporated and thereafter to a
kiln 17, e.g. a rotary kiln, wherein the particles are
!sintered to form high strength spheres suitable as prop-
ping agents. The firing process taking place in the kiln
17 is conducted under the same conditions as those used
in the prior art processes in which an agglomeration has
~een performed in a mixing apparatus.
; The size limits for the product fractions separa-
ted in the sieving unit 13 must be fixed with due
regard to the fact that in the s~bsequent firing pro-
cess in the kiln 17 a substantial shrinkage takes
- place. The extent of this shrinkage depends on the ori-
gin of the original bauxite and may typlcally clmount to
app. 25% on linear basis.
¦ As it appears the process may be p~rformed on a
continuous basis and it ls very suitable for being
; automatically controlled using a rninirnurn of manpower.
As mentioned above the resulting propping agent
!~ 30 has higher density and higher crushing strength than
obtained in the embodiment examples of the above men-
tioned German application. The crush~g strength is evalu-
ated by a method in which the fraction between app. 600
microns and app. 700 microns is placed in a 1 5/8 inch
diameter steel cylinder, and pressuré is applied to the
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sample through a plunyer fitting the -top o~ -the cylirlder
according to the following schedule: 1 minute to 700
kg/cm2, 3 minutes hold at th.is pressure and 1 mlnute
down to 0 pressure. Afterwards the amoun-t o material
having a particle size below 600 microns is measured
and expressed as % of the total amount. The result ls
recorded as the weight ~ crushing loss.
The inven-tion is further elucidated by means of
the following embodiment example.
10 Example 1
The process is carried out in a plant correspon-
ding to the one illustrated on the drawing.
In the feed tank a feed suspension is prepared
from water, fresh bauxite, recycled bauxite dust from
15 the collector unit and the below indicated auxiliary
agents in amounts giving a total solids content of the
feed~suspension of app. 50% by weight, which solids con-
sist o, ca.93% bauxite, 1% polyvinylalkohol and.0,3~ ammo-
nium citrate. This feed is in an amount of 4000 kg/hour
atomized through the pressure nozzles 3 in a fluid bed
unit in principle designed as shown on the drawing and
having a fluidizing area oE 3 m2.
The velocity of the Eluidi.zing air is 1.2 m/sec,
- and the inlet tempera-ture o the alr :Ls 550C while the
outlet temperature is 70C. Recycled mater:La:l lntroduced
: through the powcler inlet 10 amount~ to 1700 kg/hour. The
height of the fluidized particle layer 5 is app. 35 cm.
The average residence time of the particles in the
fluidi~ed layer may under these conditions be estimated
to 15 minutes.
. Through the outlet 11 material is withdrawn
in a quanti~y of 3400 kg/hour, which by sieving is sepa-
rated in an oversize fraction having a particle si.ze
above 2.1 mm (50 kg/hour), a coarse product fraction
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having a particle size between 1.2 and 2.1 mm (300 kg/
hour), a fine product fraction having a particle size
between 0.6 and 1.2 mm (2450 kg/hour) and an undersize
fraction having a parti~le size below 0.6 mm ~600 kg/
hour).
In the collector unit 9 which is a bag ilter
300 kg/hour entrained particles are collected and re-
cycled to the feed tank l.
The total amount of the oversize fraction togethex
with 400 kg/hour o the ine product fraction is ground
in a grinding unit having a sieve of mesh size 3~U mi-
crons,and together with the undersize fraction led to
the fluid bed unit as seed or nuclei particles. 650 kg/
hour fine product fraction is recycled to inlet 10 with-
out prior grunding.
The remaining material from the product fractionsis led through an oven in which the remaining moisture
and organic additions ~app. totally 4% by weight) are
removed and afterwards the sintering is perormed in a
rotary kiln at a temperature of app. 1500C. The re-
sidence time of the particles at this tempera-ture i5
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app. 10 minutes.
The sintered particles is subject to a ~urther
sieving operation to assure thak substantially all the
25 product has a particle size between 0.4 and 1.5 mm. The
sphericity of the particles is excellent ancl their den-
sity app. 3.8 g/cm3. q'he crushiny strength according to
the~above methocl is 1.5% by weight, for which reason the
`~ product must be regarded as being excellently suitable
~ 30 as a propping agent.
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Example 2
In this embodiment the process is performed in a
plant which only departs from the one used in Example
; 1 by having a sieving unit 13 which separates the par-
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ticles in only three frackions (viz. an oversize frac~
tion, a product ~raction and an undersize frac-tion) and
by having a collector means 9 compris ng a cyclone
collecting particles coarser than app. 100 microns and a
'wet scrubber collecting the finer particles as an aqueous
Islurry.
! Also in this embodiment the total solids content
'of the feed suspension is app. 50% by weight and the
quantity atomized is 4000 kg/hour.
10 ' Inlet air temperature is 530C and the outlet air
temperature 68C.
,I That part of the material recycled as seed to
the powder inlet 10 which consists of the undersize
,fraction, the ground oversize fraction and a part of
15 Ithe product fraction (ground) amounts to 300 kg/hour.
As in Example 1 the fluidizing air velocity is
1.2 m/sec. and the height of the fluidized layer is app.
35 cm. The average residence time is app. 20 minutes.
The material withdrawn from the fluid bed through
11 amounts to app. 2100 kg/hour and is let to the
sieving unit 13 having two screens of mesh width 1.5
and 0.6 mm. resp. Thereby is obtained 30 kg/hour oversize
fraction, 2030 kg/hour product fraction and 40 kg/hour
undersize fraction.
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25 , 230 kgyhour of the product fraction is together
,with the oversize frac-tion cJround to a partLcle slze less
than 600 microns ancl recycled together wlth the under-
size ~raction as mentioned above,.
, In the above mentioned cyclone 100 kg/hour par-
; 30 ticles having a size above 100 microns are collected and
recycled to fluid bed inlet 10, while in the wet
~ scrubber connected to the cyclone 200 kg/hour particles
,, of an average size below 100 microns are collected and
' recycled as a slurry to the feed tank 1.
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The product fraction not recycled which amoun-ts
to 1800 kg/hour is dryed and sintered as descrlbed in
Example 1. The crushing strength is 1.7% by weight and
the density F 3-8 g/cm .
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