Note: Descriptions are shown in the official language in which they were submitted.
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CENTRIFUGAL SEPARATOR AND METHOD FOR SEPARATING
FIELD OF THE INVENTION
The present invention relates to a centrifugal separator and a method for
controlling a radial level of an interface layer in a centrifugal separator.
BACKGROUND TO THE INVENTION AND PRIOR ART
A centrifugal separator and a method for controlling a radial level of an
interface
layer in a centrifugal separator are known from WO 00/37177 Al, which
describes
control equipment for a centrifugal separator for separating a light liquid of
relatively low density and a heavy liquid of relatively high density from a
mixture
containing these two liquids. The liquids may, for instance, be oil and water.
The
control equipment is intended for a centrifugal separator comprising a rotor,
which
is rotatable around a rotational axis and forms an inlet for said mixture and
a
separating chamber, which communicates with the inlet and which has a radially
inner zone and a radially outer zone, said zones being adapted during a
separating operation to contain separated light liquid and separated heavy
liquid
respectively.
A centrifugal separator of this kind may have outlets configured in several
different
ways for the separated liquids. Thus, the rotor may be provided with so-called
overflow outlets for both of the liquids or an overflow outlet for one liquid
and
another kind of outlet for the other liquid. An outlet of such other kind may
be
constituted by, for instance, a non-rotatable so- called paring member or by
nozzles situated in the surrounding wall of the rotor. Nozzles are used as a
rule
where the mixture supplied contains not only said two liquids but also a
relatively
large amount of solids which are heavier than the two liquids. The separated
solids together with part of the separated heavy liquid may thus be discharged
through the nozzles situated at the periphery of the rotor, whereas the
separated
light liquid is removed from a central part of the rotor via an overflow
outlet or a
paring member. In these cases the rotor may also form a space which
communicates with the radially outer zone of the separating chamber in such a
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way that during a separating operation it will contain separated heavy liquid
(but
not separated light liquid). An excess of separated heavy liquid which does
not
leave the separating chamber via said nozzles can therefore be removed from
the
rotor via this space.
Another type of centrifugal separator in which solids as well as two different
liquids
can be separated is a so-called decanter centrifuge. In a centrifugal
separator of
this kind there is in the rotor a so-called sludge conveyor, which is adapted
to
transport separated solids along the surrounding wall of the rotor to a sludge
outlet. The sludge outlet is often situated at a level in the rotor which is
radially
within the level of the outlets for the two separated liquids.
During a separating operation in a nozzle centrifuge of the kind described
above
or in a decanter centrifuge having a sludge conveyor, it may be difficult to
maintain at a predetermined radial level an interface layer formed in the
rotor
between the separated liquids therein. The reason for this is that an
uncontrollable
amount of separated heavy liquid per unit of time often leaves together with
the
separated solids via the sludge outlet of the rotor. If this uncontrollable
amount of
heavy liquid would exceed the amount of heavy liquid, which per unit of time
is
introduced into the rotor with the mixture to be treated therein, the
interface layer
in the separating chamber between light liquid and heavy liquid will move
radially
outwards, and separated light liquid will in the end be lost together with the
separated solids when the latter leave the rotor via the sludge outlet.
A particular separating operation in which this has caused problems is the
removal of sand and water from oil in connection with oil recovery from so-
called
oil sands. In this context, nozzle centrifuges are used in at least two
separating
steps.
In a first separating step a mixture of oil, water, solvent and sand residues
is
introduced into a nozzle centrifuge and, in addition to the mixture, an amount
of
water is supplied to the centrifuge. The water supplied is added to ensure
that the
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interface layer formed, in the rotor's separating chamber, between oil and
water
shall not be displaced radially outwards. Such a radial displacement of the
interface layer might otherwise take place after a number of hours of
operation
because of said nozzles becoming worn by the outflowing sand and therefore
releasing more water per unit of time than at the beginning of the separating
operation. After the first separating step, the oil still contains not only
solvent but
also residues of sand and water. This oil is conducted out from a central part
of
the rotor via a paring member and is pumped on to another nozzle centrifuge to
undergo a second separating step.
To achieve as good separating results as possible, special control equipment
has
been developed for controlling the separating operation in the first and
second
separating steps. The special control equipment is described as prior art in
WO
00/37177 Al. That control equipment makes it possible to avoid continuous
addition of excess water to the mixture being introduced into the centrifugal
rotor.
Instead, water is introduced into the separating chamber of the rotor ¨ only
when
needed and only in a required amount ¨ via a space of the kind previously
described, i.e. a space communicating only with the radially outer zone of the
separating chamber. Water is also removed from the rotor via the same space
during periods when an excess of water enters with the oil which is to be
cleaned,
which excess cannot leave the rotor through the sludge outlet nozzles.
Said control equipment includes a pressure vessel for water, the lower part of
which communicates via a conduit with a liquid transfer member, situated in
said
space in the rotor of the centrifugal separator, for the introduction of water
into or
discharge of water from the rotor. In the upper part of the pressure vessel a
gas
pressure is maintained (usually by means of nitrogen gas) and its magnitude is
continuously controlled on the basis of the amount of water present at the
time in
the pressure vessel, so that the liquid pressure at the bottom of the pressure
vessel, and therefore in the conduit via which the pressure vessel
communicates
with said space in the centrifugal rotor, is always kept constant at a
predetermined
value.
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This constant value of the liquid pressure in said conduit corresponds to a
desired
radial level in the separating chamber of the rotor for the interface layer
therein
formed between separated oil and separated water. If the interface layer moves
radially outwards from the desired level, the pressure drops in said space in
the
rotor, with the result that water is pushed from the pressure vessel into the
rotor
via the conduit until the interface layer returns to the desired radial level.
A level-
detecting member in the pressure vessel is adapted to initiate when necessary
a
supply of new water to the pressure vessel so that it never becomes empty of
water.
If the interface layer in the separating chamber of the rotor starts moving
radially
inwards from the desired level, the pressure in said space in the rotor
increases
and excess water is pushed from this space into the pressure vessel via said
conduit. When the liquid level in the pressure vessel has risen to an upper
limit
level, a bottom outlet of the pressure vessel opens to release water
therefrom.
According to WO 00/37177 Al, said control equipment is expensive, complicated
and bulky. Therefore WO 00/37177 Al describes a less expensive and a more
simple control equipment for a centrifugal separator of the kind described
above.
That control equipment includes a device for supply of a control liquid which
is of
higher density than said light liquid. The supply device has a pressure source
for
delivering pressurised control liquid, and a supply conduit which at its one
end is
connected to the pressure source in order to receive pressurised control
liquid
and at its other end is connected to a liquid transfer member in order to
introduce
pressurised control liquid into the rotor. The supply device is adapted upon
need
to supply control liquid to the rotor in only such amount per unit of time as
is
required to prevent an interface layer formed in the separating chamber
between
separated light liquid on the one hand and separated heavy liquid or control
liquid
on the other hand from moving radially outwards from a predetermined radial
supply level. The control equipment also includes a device for discharge of
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separated heavy liquid and/or control liquid from said space in the rotor. The
discharge device has a discharge conduit which is adapted, when the rotor is
supplied with an excess of heavy liquid, to discharge separated heavy liquid
and/or control liquid from the rotor through said discharge conduit in such
amount
5 per unit of time as is required to prevent said interface layer from
moving radially
inwards from a predetermined radial discharge level.
The control equipment described in WO 00/37177 Al is characterised in that the
discharge device is arranged to discharge liquid from said space in the rotor
a
different way than through said supply device. This control equipment differs
from
the control equipment previously described in that the pressure source in the
supply device is not integrated in the discharge device, i.e. the control
equipment
has respective separate conduits for supply and discharge of liquid. The
separated heavy liquid and/or control liquid leaving the rotor therefore need
not be
accumulated at an elevated pressure and no pressure vessel is needed.
Consequently, there is also no need for a system for compression of gas and
for
control of the pressure of such gas. Instead, the pressure source may be
constituted by a simple liquid pump and the whole control of the supply of
control
liquid and discharge of separated heavy liquid and/or control liquid may be by
means of so-called constant pressure valves. Any container needed for a buffer
amount of control liquid may be free of pressure and common to several
centrifugal separators.
Although the control equipment is thus simplified, it still has certain
disadvantages. The equipment includes a double set of components in the form
of
respective separate supply and discharge conduits with constant pressure
valves.
There is therefore a need to further simplify the control equipment for the
centrifugal separator.
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SUMMARY OF THE INVENTION
An object of the present invention is to provide a centrifugal separator and a
method of the initially described kind which comprise simple, inexpensive and
compact control equipment.
This object is achieved with the initially described centrifugal separator
which is
characterised in that the pressure source is a variable speed pump, wherein
said
means for detecting the interface layer level is arranged to communicate with
means for controlling the speed of the pump in such a way that the supply and
discharge respectively of the liquid in said space take place in such amount
per
unit of time that the interface layer in the separating chamber is within said
predetermined inner and outer radial levels.
The control liquid used may for example be the heavy liquid (water). The
invention
therefore does not require the control liquid to be in some form other than
the
heavy liquid, although that is also possible. All that is required is that the
control
liquid be of higher density than the light liquid.
The object is also achieved by the method initially described which is
characterised in that the liquid is supplied to and discharged from said space
by
means of a variable speed pump as said pressure source, the speed of the pump
being controlled, by the interface layer level detected in the separating
chamber,
in such a way that the supply and discharge respectively take place in such
amount per unit of time that said interface layer in the separating chamber is
within said predetermined inner and outer radial levels.
The invention differs from the first of the previously described known forms
of
control equipment in that the separated heavy liquid and/or control liquid
need not
be accumulated at an elevated pressure, i.e. there is no need for any pressure
vessel with systems for compression of gas and control of the pressure of such
gas.
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The invention differs from the second of the previously described known forms
of
control equipment in that neither the separate discharge conduit nor said
constant
pressure valves are necessary. According to the invention, supply and
discharge
of control liquid take place via the same conduit. The conduit with the
variable
speed pump is thus adapted to allow liquid flow in both directions, i.e. in
both a
supply and a discharge direction through the pump. The speed control of the
pump eliminates the need for said constant pressure valves, since the speed
control is itself adapted to control the liquid pressure in said conduit in
such a way
that the amount of control liquid supplied and discharged respectively per
unit of
time is such that said interface layer is within said predetermined interface
layer
levels.
A further object of the present invention is to provide a centrifugal
separator with
control equipment which in a simple and effective way controls said level of
the
interface layer in the separating chamber.
A further object of the present invention is to provide a centrifugal
separator with
control equipment which in a simple and effective way detects the interface
layer
level in the separating chamber.
A further object of the present invention is to provide a centrifugal
separator with
control equipment which in a simple and effective way controls the pump for
delivery of pressurised control liquid.
According to an embodiment of the present invention, the pump is a reversible
pump. Reversing the pump makes it possible for the liquid to be also pumped
from the space in the discharge direction, thereby achieving quicker control
when
the level of the interface layer is within the inner predetermined radial
level. It
should be noted, however, that the pump need not be reversible for discharge
of
control liquid. The pump may for example be a variable speed rotary pump, e.g.
a
centrifugal pump, adapted to allow backflow of liquid. The level of the
interface
layer in the separating chamber depends on the speed of the pump, wherein a
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lowering of the speed of the pump (or having the pump at standstill) will
cause the
interface layer to move radially outwards as a result of the liquid pressure
from the
separating chamber overcoming the counterpressure from the pump, thus
discharging the liquid from said space.
According to a further embodiment of the invention, said means for controlling
the
pump comprises a frequency converter. The frequency converter makes it
possible for the speed of the pump to be controlled in an energy-efficient and
simple way.
According to a further embodiment of the invention, said means for detecting
the
interface layer level in the separating chamber comprises a pressure sensor
arranged to detect a change in the liquid pressure in the space, which liquid
pressure depends on the level of the free liquid surface in the space. As the
space
is in liquid-transferring communication with the separating chamber via its
radially
outer zone, the level of the interface layer in the separating chamber will
affect the
free liquid surface in the space. If the interface layer moves radially
outwards, the
free liquid surface in the space will also move radially outwards, and vice
versa. If
the free liquid surface in the space moves radially outwards, the liquid
pressure in
the space will drop (at a given radial level). In this way the interface layer
level in
the separating chamber can easily be determined via the free liquid surface of
the
space. Alternatively, a device may be provided for detecting the radial
position of
the free liquid surface in the space. In all cases a detection operation of
this kind
refers to detecting the radial position of the interface layer formed in the
separating chamber.
According to a further embodiment of the invention, said conduit is in liquid
pressure transmitting communication with the space via the liquid transfer
member, and the pressure sensor is arranged to detect a change in the liquid
pressure in the conduit between the pump and the liquid transfer member. In
this
case the liquid pressure in the conduit provides a measure of the radial level
of
the free liquid surface in the space.
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BRIEF DESCRIPTION OF THE DRAWING
The invention is explained in more detail below by a description of an
embodiment
with reference to the attached drawing.
Fig. 1 discloses schematically a longitudinal section through a rotor which
forms
part of a centrifugal separator, and control equipment according to the
invention.
DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION
Figure 1 discloses, in an example of an embodiment of the invention, a
centrifugal
separator which comprises a rotor having a lower part 1 and an upper part 2,
which parts are connected to one another by means of a locking ring 3. The
rotor
is situated uppermost on a vertical driveshaft 4 which is connected to the
lower
rotor part 1 and rotatable around a rotational axis R.
Within the rotor there is a distributor 5 which divides the rotor interior
into a central
inlet chamber 6 and an annular separating chamber 7 extending around the
distributor. The distributor 5 rests on a central portion of the lower rotor
part 1 via
radially and axially extending wings (not shown) which are distributed around
the
rotational axis R of the rotor. The inlet chamber 6 communicates with the
separating chamber 7 via channels 8 situated between said wings. A stationary
inlet pipe 9 extends from above axially into the rotor and opens in the inlet
chamber 6. Within the separating chamber 7, a stack of conical separating
discs
10 is kept axially in place between the upper part 2 of the rotor and the
lower part
of the distributor 5. Each separating disc 10, like the lower part of the
distributor,
has at its outer periphery a number of recesses 11 distributed around the
rotational axis R and situated axially in line with one another. Although the
embodiment disclosed comprises separating discs and a distributor with
recesses
at its outer peripheral edge, it should be noted that rotors also exist which
exhibit
separating discs with a distributor which has recesses or holes radially
within its
outer peripheral edge (i.e. holes a short distance in on the conical surface
of the
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separating disc), which recesses or holes are likewise distributed around the
rotational axis R and situated axially in line with one another.
At the radially outermost part of the separating chamber 7 the lower rotor
part 1
5 carries several nozzles 12 distributed around the rotational axis R of
the rotor.
Each nozzle 12 has a through channel via which liquid and finely divided
solids
can be ejected from the separating chamber 7.
The upper rotor part 2 carries a central annular cap 13, which on its inside
delimits
10 an annular outlet chamber 14 open radially inwardly towards the
rotational axis of
the rotor. The outside of the stationary inlet pipe 9 supports an outlet
member 15
in the form of a paring disc which extends radially outwards into the outlet
chamber 14.
A radially inner zone 7a of the separating chamber 7 communicates with the
outlet
chamber 14 via an overflow outlet 16 formed by an annular flange which is
supported by, and situated on the inside of, the upper rotor part 2. The
overflow
outlet 16 is not necessary for the function of the rotor and might, if
desired, be
dispensed with. Alternatively, the outlet member 15 might be dispensed with,
in
which case liquid flowing out from the separating chamber 7 might leave the
rotor
directly.
An annular space 17 delimited in the lower part 1 of the rotor is open
radially
inwardly towards the rotational axis R of the rotor. The space 17 communicates
with a radially outer zone 7b of the separating chamber 7 via channels 18 and
19
and a plurality of pipes 20 distributed around the rotational axis R. Instead
of the
pipes 20, the rotor may of course be provided with other means for creating
said
connection between the radially outer zone 7b and the annular space 17. For
example, the rotor may be provided with channels integrated in the lower part
1 of
the rotor, in which case the channels will constitute extensions of the
channels 18
and 19. Alternatively, the pipes 20 may be replaced by a separate plate
arranged
at the lower part 1 of the rotor and provided with channels which connect to
the
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channels 18 and 19 of the rotor part and which create said connection between
the radially outer zone 7b and the annular space 17.
A stationary liquid transfer member 21 extends into the space 17 and is
adapted
to conduct liquid into the space 17 or conduct liquid out therefrom.
A vertical broken line 22 in the separating chamber 7 represents a certain
radial
level therein.
The centrifugal rotor is suitable for treating a mixture of oil and water and
of solids
therein suspended. The mixture is supplied to the rotor via the inlet pipe 9
and is
forwarded from the inlet chamber 6 to the separating chamber 7 via the
channels
8. The mixture is distributed, via distribution channels formed by the
recesses 11
in the separating discs, to the various interspaces between the separating
discs
10, in which the various mixture components are separated from one another.
Separated oil flows radially inwards and further out from the rotor via the
outlet
chamber 14 and the outlet member 15, whereas separated solids and water leave
the rotor via the nozzles 12.
If the amounts of water and oil which respectively leave the rotor via the
nozzles
12 and the outlet member 15 equal the amounts of water and oil contained in
the
mixture supplied to the rotor, a state of equilibrium occurs on the interface
layer
between separated oil and separated water. Such a state of equilibrium is
shown
schematically in the figure at the radial level 22 in the separating chamber
7. In a
state of equilibrium of the kind described it is presumed that free liquid
surfaces
establish themselves in the various chambers and spaces of the rotor at the
radial
levels indicated in the figure by small triangles. If the state of equilibrium
coincides
with a desired predetermined radial level, no adjustment of the radial level
22 of
the interface layer takes place, i.e. no liquid flows out from the rotor or
into the
rotor via the liquid transfer member 21. It should be noted, however, that the
radial
position of the radial level 22 shown in the figure need not be the state of
equilibrium formed in practice, nor the desired predetermined radial level. To
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achieve as good separating results as possible, the desired predetermined
radial
level may be somewhat elsewhere in the separating chamber (e.g. at or
somewhat radially outside the distribution channels formed by the recesses 11
in
the separating discs). It is further presumed that separated solids leave the
rotor
via the nozzles 12 without blocking them for outflowing separated water.
Depending on wear of the nozzles 12 and/or variations in the amounts of water
and oil in the mixture supplied to the rotor, however, it is in practice
impossible,
without using special control equipment, to maintain said interface layer
between
oil and water in the separating chamber 7 at the desired predetermined level,
which is hereinafter assumed to correspond to the radial level 22 shown in the
figure. Control equipment of this kind is connected to the liquid transfer
member
21 and is adapted either to supply via the latter to the rotor a variable
amount of
control liquid (e.g. water) if said interface layer in the rotor tends to move
radially
outwardly from the level 22, or to remove a variable amount of water from the
rotor if the interface layer tends to move radially inwardly from the level
22.
The control equipment comprises a device which has a pressure source in the
form of a variable speed pump 23 and a conduit 24 which is connected at its
one
end to the pump 23 and at its other end to the liquid transfer member 21. The
pump 23 is connected to a so-called VFD (variable frequency drive), i.e. a
frequency converter 25 for controlling the speed of the pump 23, which
frequency
converter 25 communicates with a sensor 26 in the form of a pressure sensor P
adapted to detect a change in the liquid pressure in the conduit between the
pump
23 and the liquid transfer member 21.
The liquid transfer member 21 may within the scope of the invention be of
various
kinds. If it is stationary, i.e. non-rotating, as illustrated in the figure,
it may with
advantage comprise an annular disc surrounding the rotor's rotational axis R
and
extending into the space 17. The liquid transfer member may form one or more
radially extending channels, or form one or more annular channels extending
around the rotational axis R (see SE 76670). In either case the channels lead
into
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the liquid which is present in the space 17. Rotation of the rotor causes a
liquid
pressure, the magnitude of which depends on the position of the free liquid
surface of the liquid body which rotates together with the rotor in the space
17.
The position of the liquid surface in the space 17 is itself influenced by any
movement of the radial position 22 of the interface layer in the separating
chamber 7 between separated oil and separated water. Thus, if the interface
layer
in the separating chamber 7 moves radially outwards, the free liquid surface
in the
space 17 also moves radially outwards, whereupon the pressure in the conduit
24
drops. Upon movement of the interface layer radially inwards, the pressure in
the
conduit 24 rises.
If the pressure in the conduit 24 tends to drop below a predetermined first
value
which corresponds to an outer predetermined radial level (somewhat radially
outside the level 22) for the interface layer between oil and water in the
separating
chamber 7, the speed of the pump 23 increases to raise the pressure in the
conduit 24 and pump water into the space 17 and further via the channels 18,
19
and the pipes 20 to the separating chamber 7. The speed increase will be
greater
or lesser depending on how low the pressure in the conduit 24 drops, and the
amount of water pumped in per unit of time will be such that the interface
layer
between oil and water in the separating chamber is kept radially within said
outer
predetermined radial level.
If instead the pressure in the conduit 24 tends to rise above a predetermined
second value which corresponds to an inner predetermined radial level
(somewhat radially within the level 22) for the interface layer between oil
and
water in the separating chamber 7, the speed of the pump 23 is reduced (or the
pump may even be reversed) to reduce the pressure in the conduit and thus
discharge (or pump) water from the space 17. Water will then flow through the
pump 23 in an opposite direction (relative to the supply direction described
above). The speed reduction will be greater or lesser (or the pump may even be
reversed) depending on how much the pressure in the conduit 24 increases,
wherein the amount of water discharged (or pumped) per unit of time will be
such
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that the interface layer between oil and water in the separating chamber 7 is
kept
radially outside said inner predetermined radial level.
The control equipment may also comprise a container 27 for control liquid
(water)
which is maintained in a desired quantity and at a desired temperature. To
this
end, the container 27 is provided with an inlet conduit, an outlet conduit, a
float,
valves controlled by the float, and a warming device (not shown in Fig. 1).
Such a
container is known per se from WO 00/37177 Al (see Fig. 6). The pump 23 is
arranged to use the container 27 as a control liquid reservoir in such a way
that
control liquid is both supplied to and discharged from the container 27 upon
adjustment of the interface layer level 22 in the separating chamber 7.
Control
liquid supplied to the container 27 may thus be reused.
Any suitable pump desired may be used for the transport of water from and to
the
space 17, subject to the pump being adapted to allow flow in both directions.
Such
pumps are well known to one skilled in the art and are therefore not described
here.
In a similar way, the pressure sensor 26 for detecting the pressure in the
conduit
may be replaced by other means for detecting the level of the interface layer.
Instead of said pressure detection, means may be provided for detecting the
radial
position of the free liquid surface in the space 17. Also conceivable are
other
indirect means of detecting the interface layer level which detect at least
one
parameter related to the position of the interface layer and which, on the
basis of
said parameter or parameters, calculate or otherwise determine the radial
position
of the interface layer. In all cases, a detection operation of this kind
refers to
detecting the radial position of the interface layer formed between oil and
water in
the separating chamber 7. If so desired, it is of course also possible to
provide
means for direct detection of the radial position of the interface layer.
Control equipment according to the invention will of course also work in
conjunction with a hermetically closed centrifugal rotor, i.e. a centrifugal
rotor in
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which a space 17 is intended to be completely filled with liquid and to
communicate with the interior of a stationary liquid transfer member which
seals
against the rotatable centrifugal rotor.
5 The
invention is not limited to the embodiment disclosed, but may be varied and
modified within the scope of the claims set out below.
