EP1958698B1 - Method for operating a centrifuge - Google Patents

Abstract

A suspension of the product is introduced into a working region (60) through a drive shaft (20), a drum being rotated continuously during filling. The suspension is centrifuged. The product is dried, during which the drum rotates continuously. A fluid (the drying gas) is injected by nozzle(s) through the annular space (18) and into the interior (60) of the drum (10). The speed of the drum is controlled during drying, such that product cake does not collapse. During centrifugation, the internal pressure in the drum is raised.

Description

The present invention relates to a method for the operation of a centrifuge, such as this example EP 045 4 045 A is known.

Centrifuges are well known in the art. They are primarily used in the chemical, pharmaceutical and food industries, in suspensions, i. Substances with a liquid and a solid content to separate the solid phase from the liquid phase and to dry.

In general, conventional centrifuges comprise a drum with a filter disposed in the drum. The filter can be designed as a rigid metal filter. The space between filter and drum wall is also referred to as annulus. The area inside the filter is called working space.

In conventional centrifuges, the suspension is first filled in the working space. This is conventionally done by the drive shaft, which is hollow, so that it can be used as a filling shaft. The drive shaft is further firmly connected to the drum base and serves to drive the drum. Usually, the drive shaft is mounted horizontally.

The suspension is filled with rotating drum in the working space. By acting on the suspension Forces in the radial direction, such as the centripetal force, or the resulting inertial forces, such as centrifugal force, the suspension is pressed outwards to the filter. With a correspondingly high centrifugal force, a stable liquid ring is created. This creates a suspension ring on the filter. The liquid phase now passes through the filter into the annulus and is discharged while the solid phase remains in the working space.

In conventional centrifuges, the solid phase of the product adheres firmly to the filter after the liquid phase has escaped. The solid phase may have a residual liquid content of up to 30%. The firmly adhering to the filter product is called in this state, cake or ring cake, product cake or filter cake.

The centrifuged product with a high residual liquid content in the form present after centrifuging is generally not optimally suitable for further transport for a further process step "drying". It has proved to be particularly advantageous to dry the product directly in the working space. In this way it is prevented to have to spend the still wet and only cumbersome transportable product via a transfer unit in a drying room. For toxic products, the risk for the personnel involved also decreases. Centrifuges in which a product is centrifuged and dried in the same working space are also referred to as centrifugal dryers.

In conventional centrifugal dryers, the cake must be blasted off the filter before drying. For this purpose, shot nozzles and drum bottom openings are provided, which open into the annular space. The annulus itself is divided by webs into several sections, each section having a drum bottom opening having. Furthermore, it is conventionally provided that the firing nozzles can be moved to the drum base openings from the outside. The shot nozzles now inject a generally gaseous fluid at high pressure into the annulus. The fluid now moves in the opposite direction through the filter and dissolves the solid phase of the product pressed by the centrifugal forces into the filter from the filter. This process is also called breaking the filter cake. Optionally, several firing nozzles may be provided so that they inject the fluid into the annulus at the same time, or it may also be provided only a shot nozzle, which successively injects the fluid into the individual sections and thus peel off the filter cake piece by piece.

After the filter cake has been broken off, the product is dried. The drying is conventionally carried out either by fluidized bed drying or fixed-bed drying.

After drying, the dried product, which now usually takes the form of a powder, can be removed from the working space and processed further.

In the conventional methods described above, however, problems occur in the processing of certain products. Especially for products with a broad range of grain sizes and a high proportion of fines, centrifuging is made considerably more difficult.

Already during filling a sedimentation of the larger product parts takes place. Due to their different ratios of mass to surface, the larger proportions of product rapidly move radially outward to the filter. However, the fine fraction first floats in the liquid and settles more slowly to the outside of the filter. In the process, the fine product components clog the interspaces between the larger product parts, so that it is frequently not possible for the liquid phase to flow out through the capillary between the larger product portions. The liquid phase then flows during centrifugation only very slowly or not at all. An increase in the rotational speed of the drum does not solve this problem either. For problematic products, this results in a residual liquid content of the product of up to 70% after centrifuging.

To solve the above problems, a method for operating a centrifugal dryer according to claim 1 is proposed.

In the method according to the invention, a fluid is already injected during the centrifuging by means of the firing nozzles through the annular space in the drum. This loosens the product during centrifugation and prevents the product from sticking firmly to the filter. This not only prevents a product having a broad grain size spectrum from being clogged, but also prevents the product from sticking to the filter in the ring form. Thus, the duration of centrifugation is significantly shortened because the liquid phase can flow off faster due to the more porous product.

Of course, the centrifuging step according to the invention with constant loosening of the product cake can also be used separately in any method for the operation of a centrifuge. Thus, the centrifuging step may in particular also be used prior to any conventional drying step, such as conventional fixed bed drying or conventional fluidized bed drying, in any centrifuge manner.

Of course, even during centrifugation by pumping a suitable gas through the Filling shaft, the internal pressure of the drum can be increased to accelerate the escape of the liquid phase.

For injecting the fluid into the annulus, at least one firing nozzle is provided for this purpose. In one embodiment, two shot nozzles are used. The firing nozzles are on the one hand at the so-called 6 o'clock position, d. H. at about the bottom of the drum and at the 7 o'clock position, d. H. slightly laterally offset from the lowest point, arranged. In one embodiment of the invention, the firing nozzle is offset at an angle of approximately 30 ° at the 7 o'clock position relative to the firing nozzle at the 6 o'clock position.

The 6 o'clock position is therefore particularly advantageous because the acceleration due to the radial movement of the product still adds up to the acceleration occurring radially outwards. The outward forces are therefore greatest at the 6 o'clock position. Therefore, working at this point with the highest pressure when shooting the fluid to loosen up the cake and the best increase in porosity can be achieved.

In one embodiment of the invention, the annular space is divided into 12 sections, each having a drum base opening in the form of a slot. During one revolution of the drum, the two firing nozzles can inject the fluid into the same drum bottom opening or else into different drum bottom openings. When the firing nozzles inject the fluid into the same drum bottom opening, this means that during a clockwise rotation of the drum, first the shot nozzle at the 6 o'clock position injects the fluid into the particular drum bottom opening and thereafter when the drum has moved on and the particular drum bottom opening in front of the firing nozzle is at the 7 o'clock position, the firing nozzle at the 7 o'clock position injects the fluid into the particular drum bottom opening. In this way, it can be ensured that enough fluid is injected into the drum and a proper swirling effect occurs.

It should be noted at this point that the direction of rotation of the drum and the representation of the firing nozzle positions has been selected by time for purposes of simple illustration and is not to be considered as limiting. The direction of rotation of the drum and the exact position of the firing nozzles can vary and always depends on the viewing direction.

In one embodiment of the method, it can be provided that the firing nozzles in each case inject the drum in each case in staggered drum bottom openings. It can be provided that the fluid is injected in each case displaced by a drum base opening with each drum movement. In this way, it is ensured that the product is whirled over the entire circumference of the drum.

In one embodiment of the method, the drum rotates during the filling of the product suspension in the drum. The injection of fluid by means of the firing nozzles can then be done already at the beginning of filling.

As will be explained in more detail below, according to the invention, during the injection of fluid, the drum rotates at a lower rotational speed than is normally the case in conventional centrifuges in the corresponding process steps. With a drum diameter of 400 mm, the speed during injection is about 150 rpm. At this speed, however, no stable liquid ring can build up.

Due to the lower viscosity of the liquid suspension portion, the shear forces from the muddy solid portion, which rapidly builds up into a ring during filling, can be poorly transferred to the liquid portion. Within the already established solid ring therefore forms a kind of lake from the filled suspension. In the method according to the invention, the loosened solid ring is able to entrain a thick layer of the liquid fraction over a certain angular distance before it breaks off again from the solid ring. An approximately 1 mm thin suspension layer but remains over the entire solid ring.

Due to the fluidization of the suspension lake due to the rotation of the drum, a sedimentation of the suspension and a separation of the product components with larger particle sizes of the fines is prevented. This layer formation of the different grain sizes takes place during conventional filling during filling and sits down during centrifuging. In the method according to the invention, however, this is prevented from the beginning.

Due to the permanent fluidization of the solid ring by means of the injected fluid of the solid ring is much more permeable to the overlying suspension ring. Since the overlying suspension ring only has a thickness of about 1 mm and the solid ring is constantly fluidized, the suspension ring filtered off very quickly. As a result, no sedimentation can occur in the liquid ring.

Furthermore, it can be provided that the product during the entire process, ie, that the product during filling, centrifuging and drying, by means of the injected through the firing nozzles in the working space fluid is loosened. The filling step may then proceed smoothly to the centrifuging step, then the centrifuging step may then smoothly proceed to the drying step.

As already stated, the drum speed during filling, during centrifuging and during drying is chosen so that the product cake is retained and does not collapse even in spite of the fluid injected by means of the firing nozzles. Only when homogenizing a lower speed is selected so that the product falls before reaching the apex of the drum.

Furthermore, the possible speeds are limited by the fact that with increasing speed, the residence time of the drum bottom openings before the firing nozzles at some point is too short to inject a necessary amount for fluidizing fluid quantity. The amount of fluid is then too small to whirl the product cake in the desired manner.

A minimum speed thus always results from the point at which the ring shape of the product cake is no longer preserved and the product cake collapses to a maximum speed results from the amount of fluid that can give the firing nozzles in a given period, and the Shape of the drum bottom openings and the associated residence time of the drum bottom openings in front of the firing nozzles.

For a drum with a diameter of 400 millimeters and a subdivision of the annulus into twelve sections, wherein each section has a drum base opening in the form of a slot, the following possible drum speeds for the corresponding method steps could be determined.

However, it should be expressly pointed out at this point that the determined rotational speeds can also be transferred to any other drum sizes by means of mechanical principles. The method according to the invention can in principle be applied to centrifugal dryers of all sizes and with any subdivision of the annular space.

wobei v die Umfangsgeschwindigkeit und r der Radius der Kreisbewegung ist. Für v gilt des weiteren $$\mathrm{v}\mathrm{=}\mathrm{\omega }\mathrm{*}\mathrm{r}\mathrm{.}$$

For the centripetal acceleration a during the circular motion, the following applies: $$\mathrm{a}=\sqrt{\frac{{\mathrm{\nu }}^2}{r}}.$$

where v is the peripheral speed and r is the radius of the circular motion. For v further applies $$\mathrm{v}\mathrm{=}\mathrm{\omega }\mathrm{*}\mathrm{r}\mathrm{,}$$

Thus results for the centripetal acceleration $$\mathrm{a}\mathrm{=}{\mathrm{\omega }}^2\mathrm{*}\mathrm{r}\mathrm{,}$$

With the speeds specified here and the drum radius of 200 millimeters, the acceleration acting on the product can thus be determined approximately and thus at least approximately calculated back to the necessary speeds for other drum radii. The necessary rotational speed ω ₂ at a drum radius of r ₂ results at a drum radius of r ₁ equal to 200 mm and the rotational speeds ω ₁ given here below $${\mathrm{\omega }}_2={\mathrm{<figure-callout\; id="1"\; label="\omega "\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">\omega </figure-callout>}}_1\mathrm{*}\sqrt{\frac{r1}{r2}}$$

The rotational speeds indicated here for a drum diameter of 400 mm are thus transferable at least approximately without problems to other drum sizes by means of the equation (4).

As an alternative, the accelerations acting on the product cake are also given in g.

For the rotation of the drum with permanent loosening of the product cake by injecting a suitable fluid by means of the firing nozzles through the annulus into the working space a suitable speed of 120 to 150 revolutions per minute was determined with a drum diameter of 400 mm. This corresponds to an acceleration of 5 g acting on the product cake.

The speed of 150 revolutions per minute or the acceleration of 5 g can be used during filling, during centrifuging and during drying.

In less problematic products that do not clog during centrifugation, of course, during filling and centrifuging with a higher speed of about 500 revolutions per minute with a drum diameter of about 400 mm are used to accelerate the centrifuging. The acceleration acting on the product cake can be up to 55g during filling and up to 600g during centrifuging. For very coarse-grained products, it is even possible to accelerate up to 2000g on suitable centrifuges.

For problematic products that clog quickly and significantly slow down centrifugation, the rate of discharge during centrifugation could be doubled by the method of the invention. Thus, an accelerated and therefore economically advantageous processing of the product is possible.

Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the respectively specified combination but also in other combinations or in isolation, without departing from the scope of the present invention.

Figur 1

zeigt eine seitliche Querschnittsansicht eines Zentrifugentrockners, auf dem das erfindungsgemäße Verfahren ausgeführt werden kann.

Figur 2

zeigt eine schematische Frontalansicht einer Trommel mit möglichen Positionen der Schussdüsen und der Trommelbodenöffnungen, um das erfindungsgemäße Verfahren auszuführen.

Figur 3

zeigt das Korngrößenspektrum eines lediglich als Beispiels aufgeführten problematischen Produkts, das durch das erfindungsgemäße Verfahren besonders vorteilhaft zentrifugiert und getrocknet werden kann.

The invention is illustrated schematically with reference to an embodiment in the drawing and will be described below in detail with reference to the drawings.

FIG. 1

shows a side cross-sectional view of a centrifugal dryer on which the inventive method can be carried out.

FIG. 2

shows a schematic frontal view of a drum with possible positions of the firing nozzles and the drum bottom openings in order to carry out the inventive method.

FIG. 3

shows the grain size spectrum of a problematic product listed only as an example, which can be centrifuged and dried by the method according to the invention particularly advantageous.

FIG. 1 shows a centrifugal dryer on which the inventive method can be carried out. The centrifugal dryer has a drum 10 which comprises a drum shell 11 and a drum base 12 which is fixedly connected to a drive shaft 20 used for filling. The drum base 12 further has drum bottom openings 14. Within the drum 10, a metal filter 16 is arranged. Between the metal filter 16 and the drum shell 11 is an annular space 18, in which the drum bottom openings 14 open. The annular space 18 is divided in the illustrated embodiment into twelve sections, each having a trained as a slot drum bottom opening 14.

Within the filter 16 is the working area 60 in which the product is processed, ie centrifuged and dried, will. Opposite the drum base 12 of the work area 60 is closed by a baffle plate 40, which can be opened. When the baffle plate 40 is open, the product can be transferred from the working space 60 into the area 80 and removed.

Furthermore, a drain 52, through which the liquid phase of the product can flow, and an outlet 54 are provided, through which gases introduced into the working area can escape.

FIG. 2 shows a schematic frontal view of the drum 10. The direction of rotation of the drum 10 is in the present example in the clockwise direction. The vertex 70 of the drum 10 as well as a first weft nozzle 31 and a second weft nozzle 32 are also shown. The first weft nozzle 31 is in the so-called 6 o'clock position and the second weft nozzle 32 is in the so-called 7 o'clock position , The first firing nozzle 31 is offset from the second firing nozzle 32 by about 30 °. Furthermore, further firing nozzles may be provided, such as the third firing nozzle 33, which is located at the eleven o'clock position. The firing nozzles inject a suitable fluid, which is preferably gaseous, into the working area 60 through the drum bottom openings formed as elongated holes 90 through the annular space 18 and the filter 16.

As further prerequisites, the centrifugal dryer used should have a single-motor concept as a drive. This makes it possible to continuously approach all speeds between a standstill and a maximum speed of the drum. This is important in that conventional centrifugal dryers often use a twin-engine concept that includes a main and a geared motor. These are connected via a centrifugal clutch, However, only fully opens at a speed of about 160 revolutions per minute. The gear motor itself runs only up to 5 revolutions per minute. However, since the speed range between 0 and 150 revolutions per minute is currently used in the method according to the invention, this twin-engine concept is not suitable for the present method.

Furthermore, the centrifugal dryer on which the process according to the invention is carried out should have a suitable control system. In particular, the centrifuge dryer must be able to control the injections through the shot nozzles 30 in the millisecond range. The position of the drum 10 must be able to be detected in the minute range (based on the angular position). For this purpose, in particular a play-free and rigid coupling between the drive shaft 20 and the motor is necessary. By a single engine concept, this can be suitably provided.

FIG. 3 shows the grain size spectrum of a typical product that can be processed by the method according to the invention. As FIG. 3 shows, the product has a fines content of about 20%. About 20% of the product has a grain size of 10 μm or less. However, the product shown is not to be considered as limiting the use of the method of the invention. Rather, the method of the invention provides improved centrifugation and drying in all types of products.

In carrying out the method according to the invention, a product suspension is first introduced into the working area 60 in a first step of filling through the drive shaft 20 designed as a filling shaft. The drum 10 rotates continuously during filling. The speed of the drum is chosen so that the forces in Radial direction are so high that forms a ring of the product suspension on the filter 16.

Already during filling, a suitable fluid can be injected into the working space by means of the general shot nozzles designated 30. The shot nozzles 30 are moved close to the drum base, so that there is only a minimal gap between the nozzle head 38 and the drum base 12. It should be noted that the firing nozzles 30 are always fixed while the drum 10 is rotating. In order to be able to move the firing nozzles 30 as desired to the drum base 12, the firing nozzles can be made axially movable. A feed duct 34 is surrounded by a bellows 36 and the firing nozzles 30 can be moved axially by means of a suitable device.

Under a suitable fluid is basically to understand such a fluid that does not cause any chemical reactions in the product and does not damage the product in any other way. The fluid used is usually gaseous.

The firing nozzles 30 initially inject the fluid through the elongated holes in the annular space 18, from which the fluid moves through the filter 16 into the working space 60. The fluid exits the working space 60 at another location in the opposite direction again through the filter 16 and escapes through the drum bottom openings 14 and the outlet 54th

A first nozzle 31 and a second nozzle 32 inject during a first revolution in the same slot 14 'a. During the next drum revolution they inject the fluid into the next, ie offset by one, slot 14 ", etc. This ensures that the product over the entire Drum circumference is whirled up. The whirling up thus always takes place when the corresponding drum section passes the range between 6 and 7 o'clock.

During filling, an initially thin, solid-state ring forms on the filter. Inside this ring is a suspension consisting of the liquid fraction and the remaining solids, continuously adding suspension to a maximum. However, due to the low viscosity of the liquid portion and the associated poor transmission of shear forces in the suspension, the suspension can not build up into a stable ring. The suspension thereby forms a suspension lake within the solid ring. However, the shear forces are large enough that forms an approximately 1 mm thin suspension layer on the inside of the solid ring.

Due to the fluidization of the suspension lake due to the rotation of the drum, however, a sedimentation of the suspension and a separation of the product components with larger particle sizes from the fines during the filling is prevented in the inventive method.

Furthermore, by increasing the porosity of the solid ring due to the injection of fluid by means of the firing nozzles and the resulting rapid filtration of the liquid, a sedimentation of the solids content in the approximately 1 mm thin suspension ring is prevented.

Once the entire amount of suspension has been filled, the filling step is then transferred to the centrifuging step. The drum 10 rotates at a suitable speed during centrifuging, at a drum diameter of 400 mm, for example 150 rpm, or so fast that 5 g act on the cake, and the suspension is constantly loosened up by the injection of fluid by means of the firing nozzles 30. This prevents the smaller particle size of the product from being trapped between the larger particle size portions of the product and clogging the capillaries needed to complete the liquid phase. Also, the filter itself is not clogged by the fine product components, since it is regularly flowed through in the opposite direction of the fluid gas.

For example, the filling and centrifuging at a drum speed of 500 rev / min. or with a radial acceleration of 55 g, and only to dry the drum speed to 150 U / min. be lowered.

Moreover, the process according to the invention provides a shorter drying time for all types of products, ie also for those products which hitherto were considered to be non-problematic, due to the higher porosity of the product during processing. The inventive method is thus not on the example. In FIG. 3 but can advantageously be applied to all types of chemical and pharmaceutical products, as well as food and all kinds of centrifuges.

Claims (6)

1. A process for operating a centrifuge comprising a drive shaft (20) used for loading, a drum (10) which is connected to the drive shaft and has a drum casing (11) and a drum base (12), a filter (16) with a working region (60) situated within the filter, an annular space (18) formed between the filter (16) and the drum casing (11), at least one drum base opening (14) which is formed in the drum base (12) and opens out into the annular space (18), and at least one swirl nozzle (30, 31, 32, 33) which is arranged in such a manner that it injects a fluid through the at least one drum base opening (14) into the annular space (18), characterized by the following process step:

   - centrifuging a suspension of a product, wherein during centrifuging a fluid is injected into the working region (60) through the annular space (18) by means of the at least one swirl nozzle (30, 31, 32, 33).
2. A process according to claim 1, wherein the drum (10) rotates during centrifuging, as a function of a diameter of the drum (10), at a rotational speed which results in similar acceleration of the product in the radial direction to a rotational speed of approximately 150 revolutions per minute at a diameter of the drum (10) of approximately 400 mm.
3. A process according to claim 1, wherein the drum (10) rotates during centrifuging at a rotational speed which causes in the product cake radial acceleration approximately in a range of from 1 to 10 g.
4. A process according to any one of claims 1 to 3, wherein the drum (10) rotates during loading, as a function of a diameter of the drum (10), at a rotational speed which results in similar acceleration of the product in the radial direction to a rotational speed of approximately 150 revolutions per minute at a diameter of the drum (10) of approximately 400 mm.
5. A process according to any one of claims 1 to 3, wherein the drum (10) rotates during loading at a rotational speed which causes in the product cake radial acceleration approximately in a range of from 1 to 10 g.
6. A process according to claim 3 or 5, wherein the acceleration which is caused is 5 g.

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