WO2016099277A1

WO2016099277A1 - Apparatus for carrying out a physical and/or chemical process, in particular a heat exchanger

Info

Publication number: WO2016099277A1
Application number: PCT/NL2015/050890
Authority: WO
Inventors: Dick Gerrit Klaren
Original assignee: Klaren International B.V.
Priority date: 2014-12-18
Filing date: 2015-12-18
Publication date: 2016-06-23
Also published as: NL2014011B1

Abstract

An apparatus for carrying out a physical and/or chemical process in particular a heat exchanger. The apparatus comprises a reservoir provided with at least one riser in which a stream of particles in a fluid, moves between an riser inlet and riser outlet to define a first flow path, wherein the riser inlet is in open communication with a box wherein at least one distribution plate is arranged in the box for supporting a fluidized bed of particles maintained in a quasi-stationary condition. The riser outlet is coupled via at least one separator to at least one downcomer for feeding particles back from the riser outlet to the box, in which separator the stream of particles in a fluid exiting from the riser outlet is separated into a stream of particles and a stream of fluid, wherein in the downcomer, the stream of particles moves between a downcomer inlet and a downcomer outlet to define a second flow path, wherein the downcomer outlet is coupled to the box, wherein the downcomer includes an entraining device to entrain the stream of particles in the second flow path.

Description

Title: Apparatus for carrying out a physical and/or chemical process, in particular a heat exchanger The invention generally relates to an apparatus for carrying out a physical and/or chemical process in particular a heat exchanger. These apparatus, in particular heat exchangers, e.g. shell and tube heat

exchangers are known in the art and they typically comprise a reservoir provided with at least one riser in which a stream of particles in a fluid moves between an riser inlet and riser outlet to define a first flow path, wherein the riser inlet is in communication with a box wherein at least one distribution plate is arranged in the box for supporting a fluidized bed of particles maintained in a quasi-stationary condition, wherein the riser outlet is coupled via at least one separator to at least one downcomer for feeding particles back from the riser outlet, in which separator the stream of particles in a fluid exiting from the riser outlet is separated into a stream of particles and a stream of fluid, wherein in the downcomer, the stream of particles moves between a downcomer inlet and a downcomer outlet to define a second flow path, wherein the downcomer outlet is coupled to the box.

Such an apparatus is described in EP0864831, wherein a stream of particles in a fluid, e.g. glass beads, or metal particles flows from the riser outlet into a separator where the particles are, at least partially, separated from the liquid, after which the particles flow from the separator into the riser inlet through one downcomer. The natural circulation of the stream of particles in a fluid is caused and maintained by the difference in bed porosity in the risers and the downcomer.

Although the known apparatus is quite successful, a disadvantage of the known apparatus is that it is relatively large, complex and expensive to be constructed. Specifically, the known apparatus requires a minimum amount of risers and downcomers with the same diameter to ensure a stable natural circulation of fluidized of particles.

For example, in the field of heat exchangers it is especially difficult to describe and predict the fouling phenomenon in these risers. Therefore, to convince and/or to ensure the client for non-fouling operation in combination with installation of sufficient heat transfer surface to meet the required heat transfer performance, a relatively large representative test heat exchanger needs to be build, wherein the riser and/or downcomer may have a tube diameter of 20- 50 mm and a tube length of 1 - 30 m. These heat exchangers may require a minimum of, e.g. 7 parallel tubes comprising 6 risers and 1 downcomer of the same diameter to ensure a stable and continuous recirculation flow of the fluidized of particles. If the downcomer is fed with flow of particles of, e.g. only 3 or less risers, the downcomer is not supplied with enough particles and the driving force for natural

recirculation decreases, and due to the high wall friction effects at very low velocities, the flow in the downcomer looses its continuous behaviour.

The invention aims at alleviating one or more of the

aforementioned disadvantages. Thereto the invention provides for an apparatus according to the preamble of claim 1, wherein the downcomer includes an entraining device to entrain the stream of particles in the second flow path.

By providing an entraining device in the downcomer, it can be achieved that the size and the costs of the apparatus may be decreased while the flow and/or circulation of the stream of particles and/or the stream of fluid and/or the stream of particles in a fluid may be maintained. In particular, the entraining device is arranged to displace the stream of particles in the downcomer such that the stream of particles are forced through the downcomer and less risers and/or downcomers may be used to ensure a continuous recirculation flow of the stream of particles in a fluid and/or stream of particles. In addition, it may be achieved to have an operated heat exchanger which comprises at least a single riser and a single downcomer. This way, it is possible to operate on a small liquid flow often only available, use different size and type of particles and that the liquid mass flow and particle mass flow (equivalent to the particle mass flow or stream of particles in the downcomer) may be varied through the single riser independent of each other, and generate an array of data with respect to the overall heat transfer coefficient and the fouling behaviour of the system. This valuable information may be used for the further design of any heat exchanger, e.g. full-size heat exchangers.

Moreover, the stream of particles in a fluid may not be solely dependent on the drive force for natural recirculation. With the entraining device it may be achieved to have a forced recirculation of the stream of particles in a fluid. Therefore, more particles, that may be solid, may be moved through the riser, and this way, the rate of fouling removal in the riser may be increased.

The entraining device may be extending, at least partially, through the downcomer along with the stream of particles. In particular, the entraining device is situated near the downcomer inlet and extends towards the downcomer outlet, and thus forcing the stream of particles from the downcomer inlet towards the downcomer outlet. The entraining device may e.g. be an endless conveyor or a chain that moves through the downcomer. The endless conveyor or chain may be extending along the center of the downcomer and/or near a wall of the downcomer. In particular, the entraining device is provided with entraining elements such as blades, wings, flaps, comb and/or fingers to engage the stream of particles in the downcomer such that these particles are forced through the downcomer at a few cm/s, e.g. with a velocity that may range between 1- 10 cm/s and preferably about 5 cm/s. To further increase the transport efficiency of the entraining device, a flow exiting from the separator and/or paddles that may be positioned above and/or near the entraining device may be used to push the stream of particles onto and/or between the entraining elements.

The entraining device may be arranged to move with an adjustable entrain velocity, for example, with a velocity that may range between 1-20 cm/s, preferably 3-10 cm/s and more preferably about 5 cm/s or with a frequency of rotation that may, e.g. range between 40-80 rpm and preferably about 60 rpm. By regulating the entrain velocity of the

entraining device, it may be achieved to regulate the rate of fouling removal in the riser. The rate of fouling removal in the riser may be increased by increasing the entrain velocity. In particular, the rate of fouling removal may be balanced with the rate of precipitation in the riser. This way, the rate of wear of the riser may be optimized. In particular, when the rate of fouling removal equals the rate of precipitation, the wear of the riser due to e.g. the wall friction of the stream of particles in a fluid may be minimal.

The entraining device may be a rotating entraining device, preferably with a rotation axis extending along the downcomer, e.g.

extending along the second flow path. It is noted that the entraining device may also have its rotation axis transverse to the downcomer. The entraining device may then be embodied as a drum, wherein the drum may also further have entraining elements.

Preferably, the entraining device may be arranged to entrain the stream of particles along a helix like path in the downcomer. In addition, the entraining device may comprise half elliptical plates as entraining elements, wherein the plates may, e.g. be arranged on and along an axis under an angle and opposite to each other. Additionally, the entraining device may be a screw pump, also known as the Archimedes screw pump. By having a rotary positive displacement entraining device, e.g. a screw pump, relatively large amount of particles in the downcomer may be displaced within an compact volume. When the downcomer includes a positive displacement entraining device to entrain the stream of particles in the second flow path, it may be achieved that particles may be forced through the downcomer by physical contact pushforce. Positive displacement may be particularly advantageous in case of recirculation of cleaning particles that have an irregular shape, e.g. chopped steel wire, in particular abrasive particles, as it facilitates flow.

The downcomer may typically be a single downcomer. The quasi stationary bed may typically be arranged such that all or practically all particles exit the bed. The downcomer(s) may typically be used to ensure a continuous recirculation flow of the stream of particles exiting from the outlet of the riser(s). In practice, at least 80% of the particles in the apparatus is recirculated, preferably at least 90% or 95%. This is e.g. the case in an anti -fouling application of the riser pipe(s) of a heat exchanger, in which abrasive action of a stream of particles cleans fouling off the inner surface of the riser pipe(s).

Furthermore, the stream of particles in a fluid in the riser may be moving upwards or downwards between the riser inlet and the riser outlet while the stream of particles in the downcomer may be moving downwards or upwards between the downcomer inlet and the downcomer outlet.

Furthermore, the downcomer may be arranged inside or outside the reservoir. The risers and/or downcomer may be tubular shaped but it is noted that the shape of the risers and/or the downcomer is not limited to a tube and many other shapes known in the art are possible. It is further noted that in a top section of the downcomer, the stream of particles may be a moist stream of particles and/or in a bottom section of the downcomer the stream of particles may be substantially dry, i.e. the stream of particles in the bottom section of the downcomer may have a lower moisture content, e.g. 50-90% than the stream of particles in the top section of the downcomer.

The invention further relates to a use of a rotary positive displacement device, e.g. a screw pump as an entraining device in an apparatus for carrying out a physical and/or chemical process in particular a heat exchanger ,e.g in a downcomer.

Further, the invention relates to a method of moving and/or displacing a stream of particles from a downcomer inlet to a downcomer outlet, wherein the stream of particles is entrained downward with an entraining device.

The invention further relates to a single-riser tube test apparatus, in particular for measurements of the overall heat transfer performance for a representative tube for a full-size self-cleaning fluidized bed heat exchanger including a plurality of riser pipes. In such test apparatus, the particles may be embodied as cleaning particles, and the fluid may be liquid. In such test apparatus, the diameter of the single downcomer may be larger than the diameter of the single riser pipe. The entraining device may then advantageously be a positive displacement device, and may extend

substantially downwardly along the downcomer. The entraining device may be of variable entraining velocity.

The invention further relates to a method of measuring the overall heat transfer performance for a representative tube for a full-size self-cleaning fluidized bed heat exchanger, in particular using such single- riser tube test apparatus.

The invention will be further elucidated on the basis of an exemplary embodiment which is represented in a drawing. In the drawing:

Fig. 1 shows a first embodiment of the apparatus;

Fig. 2 shows an entraining device of the apparatus of Fig. 1;

Fig. 3 shows a second embodiment of the apparatus.

It is noted that the figures are merely schematic representations of a preferred embodiment of the invention, which is given here by way of non-limiting exemplary embodiment. In the description, the same or similar part and elements have the same or similar reference signs. In Fig. l is shown an apparatus for carrying out a physical and/or chemical process in particular a heat exchanger 1, comprising a reservoir 1A provided with at least one riser 2 in which a stream of particles in a fluid 3, in particular a liquid as exemplified here, moves between an riser inlet 7 and riser outlet 11 to define a first flow path F l. In the embodiment of Fig. 1, the apparatus comprises a single riser 2. The riser may be smaller in diameter than a single downcomer of the apparatus. Such apparatus is particularly suitable as a testing apparatus. The heat exchanger in Fig. l further includes a shell 4 and an inlet 5 and outlet 6 for the shell-side flow.

The riser inlet 7 may be in open communication with a box 8A comprising an extension 8 wherein at least one distribution plate 9 is arranged in the box 8A for supporting a fluidized bed of particles 3 maintained in a quasi-stationary condition. A check (ball) valve 10 prevents back -flow of the content of the riser 2. The distribution plate need not be present as a separate component, but may in practice also be embodied in, or formed by another component, e.g. the check valve. This is in particular the case in a single riser apparatus, such as a test apparatus using liquid as a fluid. The riser outlet 11 in Fig. 1 may be coupled via a feed line 12 with a separator 13 to at least one downcomer 21 for feeding particles back from the riser outlet 11.

In the separator 13, the stream of particles in a fluid 3 is substantially separated into a stream of particles and a stream of fluid. Additionally, the stream of particles in a fluid is separated from a part of the liquid which leaves the separator through line 15. Another part of the liquid 16 with the particles passes the openings 17 in plate 18 and is collected at the bottom of the separator 19 and supplied into the top section 20 of the downcomer 21. Additionally, the stream of particles 3A in the top section 20 of the downcomer 21 may be a moist stream of particles 3A, i.e. the moisture content may be lower than the stream of particles in a fluid 3, but the moisture content may be higher than the stream of particles 3A in the bottom section 30 of the downcomer 21.

In the downcomer, the stream of particles 3A moves between a downcomer inlet 20A, which may be arranged near or at the top section of the downcomer 20, and a downcomer outlet 30A, which may be arranged near or at the bottom section of the downcomer 30 to define a second flow path F2.

The downcomer outlet 30A is coupled to the box 8A with, e.g. a second feed line 52 such that particles may be fed to the box 8A. The downcomer 21 further includes an entraining device 22 to entrain the stream of particles in the second flow path F2. The entraining device as exemplified here is a positive displacement device. The entraining device extends along the downcomer 20. The entraining device extends

downwardly along the downcomer, i.e. substantially vertically.

The top section of the downcomer 20 is connected to the bottom section of the downcomer 30. The bottom section 30 may comprise a rod 31 which may be kept in a central position by positioners 32. The rod 31 may be slightly tapered, i.e. a decreasing diameter in the direction of the stream of particles 3A to reduce the wall friction between the stream of particles 3A and the wall of the downcomer 21 and prevent jamming of the stream of particles 3A. The positioner 32 arranged at the bottom of the top section of downcomer section 20 may be connected to a ring 33 which is clamped between flanges of the respective downcomer sections 20 and 30.

Near the top of the bottom section of the downcomer 30, a chamber 34 may be arranged which is separated from the downcomer 21 by a perforated cylinder 35 with holes smaller than the size of the particles of the packed bed 3A passing through the downcomer 21. Through chamber 34 and line 36 liquid, e.g. from the moist stream of particles, is extracted out of the downcomer 21, wherein the liquid joins the liquid flow passing through line 15 and are both discharged from the apparatus 1 through line 37. For flow control adjustments, line 15 is equipped with a valve 38 and a line 36 with a valve 39. The stream of particles 3A in the bottom section 30 of the downcomer 21 may have a lower moisture content than the stream of particles 3 A in the top section 20 of the downcomer 21.

Near the bottom of the downcomer section 30 a valve 40 with valve positioners 40a, a valve stem 41 and a gland 42 may be arranged. The valve 40 may be adjusted by means known in the art. For example, the valve may be adjusted with a valve adjustment mechanism 42A in line with the downcomer as shown in Fig. l, or perpendicular to the downcomer (not shown), or even not adjustable by external means, but simply spring-loaded (not shown).

After passing the valve 40, the stream of particles 3A may drop into a cone-shape collector 43. This collector 43 may also receive a fraction of the total liquid feed flow for the apparatus through line 44. The total liquid feed flow is supplied through line 45, of which another fraction is directly supplied through line 46 to the inlet channel 7. Both hnes 44 and 46 may be equipped with control valves 47 and 48. The flow through line 44 entering the collector is split in a flow 49 and 50. Flow 49 is pushing the particle to the outlet of the collector 51 and through line 52 into the riser inlet 7. Flow 50 is entering the downcomer 21 at the bottom and flowing upward in the opposite direction with the stream of particles 3A in the downcomer 21.

Further, the entraining device 22 extends at least partially through the downcomer 21 along with the stream of particles 3A. As shown in Fig. 1, the entraining device 22 extends from the top of the downcomer 20 towards the bottom 30, and specifically, in this example, the entraining device 22 is placed and/or extending in the top section 20 of the downcomer 21.

The entraining device 22 is provided with entraining elements 22A, for example, entraining blades. As an example, Fig. 2 shows an entraining device 22 comprising half elliptical plates which are mounted on and along the axle 23 under an angle and opposite of each other and fits in the cylindrical top section 20 of the downcomer. During rotation of the axle 23 is, the tilted half elliptical plates work more or less as paddles and push the stream of particles 3A in the downcomer 21 downwards.

Advantageously, these half elliptical parts can be made from any material, even very exotic ones, e.g. stainless steel, titanium or monel which may be sometimes required when using very aggressive and corrosive liquids. The efficiency per revolution for the transport of particles may be further improved by paddles 29 in the lower section of the separator 13 and clamped on the axle 23. The paddles 29 stir the stream of particles 3A in the lower section of the separator 13 and increase the charge of the volume between these tilted half elliptical plates with particles, and, consequently, the efficiency of the entraining device 22.

The entraining device 22 is arranged to move and/or operate with an adjustable entrain velocity, for example, the entraining device may operate at a velocity, for example a rotating velocity of about 60 rpm.

In Fig. 1 the entraining device is embodied as a rotating entraining device 22, preferably with a rotation axis extending along the downcomer 21, wherein the stream of particles 3A are entrained along a helix like path in the downcomer 21, in particular the top section of the downcomer 20. In Fig. 1 the entraining device 22 is embodied as a screw pump or as a entraining device comprising half elliptical plates .

Furthermore, in Fig. 1 the stream of particles in a fluid 3 in the riser 2 moves upwards between the riser inlet 7 and the riser outlet 11 whereas the stream of particles 3A in the downcomer 21 moves downwards between the downcomer inlet 20A and the downcomer outlet 30A. The downcomer 21 is in this example arranged outside the reservoir 1A.

Optionally or additionally, the apparatus 1 may be used for measurements of the overall heat transfer performance for a representative tube for a full-size self -cleaning fluidized bed heat exchanger. In case the single-tube heat exchanger is operated with a stationary fluidized bed, valve 40 should be closed and the screw pump 22 switched off, while the

downcomer 21 can be still filled over its full length with a stream of particles 3A. Particles in the riser inlet 7 and riser 2 can be fluidized by the feed flow through line 45, line 44 and/or 46. As long as the stream of particles in a fluid 3 expands up to somewhere in the outlet section 11 of the riser 2, the heat exchanger 1 operates a stationary stream of particles in a fluid and from the relevant flows and temperatures the overall heat transfer coefficient can be calculated.

When the screw pump 22 is switch on and the valve 40 is open, particles may be added to the riser 2, and the stream of particles in a fluid 3 in the outlet section 11 will continue to expand, and enter the feed line 12 and finally particles in a fluid will be discharged into the separator 13, where particles in a fluid are separated from the liquid, and the stationary situation of the fluidized bed is changed into a situation where now indeed circulation of the particles takes place.

When the valve 40 is spring-loaded, the downward force created by the screw pump 22 opens the valve and the more force by increasing the number of revolutions of the screw pump 22 the more the valve will open. The screw pump 22 may experience a reaction force which may be taken by the axial bearing 26. The radial position of the screw 22 is in this example secured by the radial bearing 27 of the axle 23 and the accurate fit of the screw in the upper downcomer section 20. The stream of particles 30 passing valve 40 are dropped into the collector 43 and a fraction of the flow entering the collector through line 44, i.e. flow 49 flushes the particles through the outlet 51 at the bottom of the collector 43 and line 52 into the riser inlet 7 of the riser 2 from where the particles are fluidized into the riser 2 and the recirculation cycle for the particles starts again. Another fraction of the flow entering the collector through line 44, i.e. flow 50 enters the opening at the bottom of the downcomer and flows upwards into the downcomer through the stream of particles which behaves as a porous plug. At the top of the bottom section 30 of the downcomer and after passing the perforated plate 35, this flow leaves the downcomer through chamber 34 where it joins with flow 16 entering the downcomer at the top and flowing downwards. The pressure difference responsible for this upward flow 50 in the downcomer is caused by weight of the stream of particles in a fluid in the riser 2. To further create a stronger upward flow in the upper part of the downcomer 20 which comprises the screw 22, line 53 may be used which connects the feed line 45 with the chamber 34. When valve 39 in line 36 is closed and valve 54 in line 53 is opened, an upward flow through the screw 22 may be created which makes it more difficult for particles in the lower part of the separator 19 to enter the screw.

The efficiency for the transport of particles by the screw may now be intentionally reduced, which may be sometimes to be preferred, e.g. in case it is desirable to operate with a very low recirculation of particles in the riser 2 or a quasi-stationary fluidized bed and still have a sufficient number of revolutions of the axle 24 for a good sealing of this axle in gland 28.

The entraining device 22 may comprise a number of tilted half elliptical plates mounted on the axle 23, and paddles 19 may be used to increase the charge of particles in the spaces between these plates to increase the transport efficiency of the entraining device 22.

Furthermore, the flow entering the separator 13 through line 12 which splits in the flow exiting the separator 13 through line 15 and the flow 16 which enters the lower section of the separator through the holes 17 in plate 18. This flow passes the screw 21 and also pushes particles in the spaces between these plates increasing the charge of these spaces with particles and improving the transport efficiency of the screw 21. The liquid flow moves downward, passes the perforated cylinder 35, enters chamber 34. It is noted that flow 16 passing the screw and entering the chamber 35 may have the same temperature as the flow 14 leaving the apparatus through line 15 and the same temperature as the flow in line 12. Moreover the flow in line 12 is substantially equal to the sum of both flows 16 and 14. However, the upward flow 50 with a different temperature than the downward flow 16 also enters this chamber and the flows 16 and 50 are mixed which influences the temperature of the total flow finally leaving the chamber 35 through line 36.

The flow through line 36 joins with the flow through line 15, where after both flows leave the apparatus through line 37 with a temperature in between the temperature of the flow in line 12 and the temperature of the flow 50 entering the bottom of the downcomer.

To measure the performance of the riser 2 the overall heat transfer coefficient of the riser 2 may be determined as a function of time. In case the value for the overall heat transfer coefficient remains constant over a long period of time (often measured over a period in hours, days or weeks) the tube is considered to remain clean, thus the scouring action of the fluidized particles on the tube wall keeps the tube clean. When the overall heat transfer coefficient does not remain constant over a long period of time, the tube suffers from fouling and then the relevant design parameters involved may be reconsidered, and changes may be made with respect to particle size, particle diameter, density of the particle material, porosity of the fluidized bed and logarithmic temperature difference, until the condition is satisfied: rate of fouling removal > rate of fouling precipitation.

The typical measurements are the mass flow and the liquid temperatures at the shell-side and the temperatures of the liquid at the tube-side, in combination with the feed and the temperature of the feed, which is the flow of the liquid in line 45, and the discharge conditions, i.e. the flow and temperature of the liquid in line 15.

In case of a stationary fluidized bed in the riser 2, with valve 40 closed and screw pump shut off, the flow of the liquid in line 45 may be substantially equal to the flow in the riser, and with this information available, the calculation for the overall heat transfer coefficient may be performed.

In case of a fluidized bed with circulation of the particles there is a transport of liquid and particles through the downcomer and because of this the liquid flow in line 45 is not equivalent to the liquid flow in the heat exchange tube 2. As a consequence, it may be necessary to apply the conservation laws for mass and energy on the complete system to be able to calculate the flow in the heat exchange tube, which then in combination with the relevant temperatures of the flow in the tube and in the shell makes it possible to calculate the overall heat transfer coefficient for the heat exchange tube as a function of time.

In Fig. 3 another embodiment of the apparatus is shown wherein the apparatus may further comprise a second separator 100 to prevent an overload of, e.g. crystals and/or scrapings in the separator 13. The second separator 100 is arranged to separate crystals and/or scrapings which were cling onto the wall of the risers and scrapped off by the stream of particles in a fluid 3. The second separator 100 may include a perforated wall 101 provided with holes with a size that is larger than the crystals and scrapings. This way, the crystals and scrapings may pass through the perforated wall while the stream of particles 3A. The crystals and/or scrapping may then leave the second separator through line 12 B which may be connected to flow 15. This way, the stream of particles in a fluid 3A may enter the separator 13 without substantially any crystals and or scrapings.

As for the purpose of this disclosure, it is pointed out that technical features which have been described may be susceptible of functional generalization. It is further pointed out that - insofar as not explicitly mentioned- such technical features can be considered separately from the context of the given exemplary embodiment, and can further be considered separately from the technical features with which they cooperate in the context of the example. It is pointed out that the invention is not limited to the exemplary embodiments represented here, and that many variations are possible. For example, the heat exchanger may comprise multiple risers and/or

downcomers wherein the heat exchanger may be a full scale heat exchanger or may be used for testing. Furthermore, in the downcomer multiple entraining devices may be arranged to entrain the stream of particles in the downcomer. Furthermore, it is noted that the downcomer 21 and/or the separator 13 may be arranged in an externally or an internally heat exchanger configuration, e.g. inside or outside the reservoir 1A.

Furthermore, it is noted that the stream of particles in the downcomer may also be considered as a packed bed of particles.

Thus is described an apparatus, in particular a test apparatus, for carrying out a physical and/or chemical process, in particular a heat exchanger, comprising a reservoir provided with at least one riser, in particular a single riser, in which a stream of particles in a fluid, in particular a liquid, moves between an riser inlet and riser outlet to define a first flow path, wherein the riser inlet is in communication with a box that in use supports a fluidized bed of particles maintained in a quasi-stationary condition, wherein the riser outlet is coupled via at least one separator to at least one downcomer, in particular a single downcomer, for feeding particles back from the riser outlet to the box, in which separator the stream of particles in a fluid exiting from the riser outlet is separated into a stream of particles and a stream of fluid, wherein in the downcomer, the stream of particles moves between a downcomer inlet and a downcomer outlet to define a second flow path, wherein the downcomer outlet is coupled to the box, and wherein the downcomer includes an entraining device, in

particular a positive displacement entraining device, to entrain the stream of particles in the second flow path, in particular with an adjustable entrain velocity. These and other embodiments will be apparent to the person skilled in the art and are considered to lie within the scope of the invention as formulated by the following claims.

Claims

1. An apparatus for carrying out a physical and/or chemical process in particular a heat exchanger, comprising a reservoir provided with at least one riser in which a stream of particles in a fluid moves between an riser inlet and riser outlet to define a first flow path, wherein the riser inlet is in communication with a box wherein at least one distribution plate is arranged in the box for supporting a fluidized bed of particles maintained in a quasi-stationary condition, wherein the riser outlet is coupled via at least one separator to at least one downcomer for feeding particles back from the riser outlet to the box, in which separator the stream of particles in a fluid exiting from the riser outlet is separated into a stream of particles and a stream of fluid, wherein in the downcomer, the stream of particles moves between a downcomer inlet and a downcomer outlet to define a second flow path, wherein the downcomer outlet is coupled to the box, characterized in that the downcomer includes an entraining device to entrain the stream of particles in the second flow path.

2. Apparatus according to claim 1, wherein the entraining device extends at least partially through the downcomer along with the stream of particles.

3. Apparatus according to claim 1 or 2, wherein the entraining device is provided with entraining elements, for example, entraining blades.

4. Apparatus according to any one of the preceding claims, wherein the entraining device is arranged to move with an adjustable entrain velocity.

5. Apparatus according to any one of the preceding claims, wherein the entraining device is a rotating entraining device, preferably with a rotation axis extending along the downcomer.

6. Apparatus according to any one of the preceding claims, wherein the entraining device is arranged to entrain the stream of particles along a helix like path.

7. Apparatus according to any one of the preceding claims, wherein the entraining device is a screw pump.

8. Apparatus according to any one of the preceding claims, wherein the stream of particles in a fluid in the riser moves upwards between the riser inlet and the riser outlet.

9. Apparatus according to any one of the preceding claims, wherein the stream of particles in the downcomer moves downwards between the downcomer inlet and the downcomer outlet.

10. Apparatus according to any one of the preceding claims, wherein the downcomer is arranged outside the reservoir.

11. Apparatus according to any one of the preceding claims, wherein in a top section of the downcomer, the stream of particles is a moist stream of particles and/or in a bottom section of the downcomer the stream of particles is substantially dry.

12. Use of a rotary positive displacement device, for example, a screw pump as an entraining device in an apparatus for carrying out a physical and/or chemical process in particular a heat exchanger, e.g. in a downcomer.

13. A method of moving a stream of particles from a downcomer inlet to a downcomer outlet, wherein the stream of particles is entrained downward with an entraining device.