GB2289524A

GB2289524A - Wooden media for filtration, heat exchange or mass exchange

Info

Publication number: GB2289524A
Application number: GB9409967A
Authority: GB
Inventors: Clive Hadfield
Original assignee: WOODEN WONDERS Ltd
Current assignee: WOODEN WONDERS Ltd
Priority date: 1994-05-18
Filing date: 1994-05-18
Publication date: 1995-11-22
Anticipated expiration: 2014-05-18
Also published as: GB2289524B; GB9409967D0

Abstract

A filtration, mass exchange or heat exchange apparatus comprises a tank (2) having an inlet and an outlet and a mass of buoyant or floatable members (1, 1A, 10) in the tank. Means are provided for passing fluid through the tank and the buoyant or floatable members (1) serve as a filter and/or a contact surface for heat or mass transfer for two or more fluids. The buoyant or flexible members preferably take the form of wooden pebbles. A process for filtration and/or mass or heat exchange utilises the apparatus. Many different shapes of member are disclosed. <IMAGE>

Description

Title: APPARATUS AND PROCESS FOR FILTRATION AND HEAT OR MASS EXCHANGE The present invention relates to an apparatus and process for filtration and heat or mass exchange.

According to the present invention, there is provided a filtration, mass exchange or heat exchange apparatus comprising a tank, a plurality of buoyant or floatable members within the tank and means for passing fluid through the tank via the pebbles.

Further, according to the present invention there is provided a filtration, mass exchange or heat exchange process comprising passing fluid through a mass of buoyant or floatable members. The buoyant or floatable members are preferably wooden pebbles.

Constructional embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figures 1 to 7 show cross-sections through a filter tank; and Figure 8 shows a cross-section through a mass and/or heat exchanger.

As shown in Figure 1 a mass of wooden pebbles 1 is placed in a filter tank 2.

The pebbles may be balls or ovoids. As shown in Figure 2, the pebbles 1A float at various levels when the tank 2 is filled with liquid 3. Preferably the mass comprises a plurality of larger pebbles and a plurality of smaller pebbles. In one example the largest of the largest pebbles has a radius of 2 cm and the smallest of the smaller pebbles has a radius of 1 cm measured at an equivalent point.

However, the pebbles could be larger or smaller. In this example the fluid density is 1.0 gm/cm2 and the pebble density is 0.95 gm/cm2, giving a buoyancy of 0.05 gm/cm2.

For a fluid of a particular viscosity there is a relationship between drag and fluid flow velocity. Generally the drag varies as the square of the velocity. For a particular viscosity and at particular flow rates the relationship may be as follows: Drag obtained in gm/cm2 Flow Velocity in cm/sec.

0.025 0.5 x 0.1 x 0.2 ;2x 0.4 2x A flow of x cm/sec (to be determined by experiment) establishes a drag of 0.1 gm/cm2 of projected area (A).

With a projected area (A) of pebbles of 12.6 cm2, volume (V) of 33.5 cm3, fluid density of 1.0 gm/cm3, the fluid displacement = 33.5 x 1.0 = 33.5 gm and gravity = 33.5 x 0.95 = 31.8 gm resulting in a buoyancy of 1.7 gms.

In Figure 3 filtration takes place with a down flow in which the normal direction of the flow of liquid is downwards and filtration takes place on the upper side of the mass or bed of pebbles. The larger pebbles have, for example, a volume (V) in cm3 of 33.5, a projected area (A) in cm2 of 12.6, a buoyancy effect (.05V) of 1.67, a downward drag (0. 1A) at one specific intermediate flow rate (which depends upon the viscosity of the fluid) of 1.26 and thus an upward difference of 0.41 whereas the smaller pebbles have, for example, a volume of 4.2, a projected area of 3.1, a buoyancy effect of 0.21, a downward drag at intermediate flow of 0.31 and thus a downward difference of 0.1. The differences effectively cause grading of the filter bed at the commencement of filtration. However, filtration can also take place with an up flow in which the normal direction of flow of liquid is upwards counter to gravity and filtration takes place on the underside of the mass or bed of pebbles. With down flow filtration the smaller pebbles are dragged to the bottom of the tank before the larger pebbles. A similar bed grading effect can be obtained in up flow filtration as shown in Figure 5 by choosing a pebble density slightly greater than the fluid density so that the smaller pebbles are first dragged to the top of the tank due to a bigger drag minus gravity factor. Figure 4 shows backwashing after down flow filtration. After up flow filtration as shown in Figure 6 backwashing may take place as shown in Figure 7 with a lowered liquid level and with gravity assisting cake detachment.

Figure 8 shows an embodiment of the invention for use in mass or heat exchange.

A mass 10 of wooden pebbles comprises a contact surface packing in a tank 12.

The pebbles are wholly or partially suspended in liquid 13 to form a contacting column. There is no need for physical supports and thus the flow restrictions of intermediate mechanical support is avoided. The density, size and shape of the pebbles may be varied and/or mixed to suit various applications. Liquid is sprayed into the tank 12 through inlet 14, passes down through the mass of pebbles and passes out through outlet 15. Gas or vapour enters through inlet 16, bubbles up through the mass of pebbles and passes out through outlet 17. The pebbles may also be used as a contact surface for heat or mass transfer for two or more liquids. For the purposes of mass and heat exchange other shapes and combinations of shapes of pebbles may be used. For example, the pebbles may be toroids, dumbbell-shaped or banana-shaped.

The advantages of wooden pebbles are low thermal conductivity compared to metals, so that heat loss and heat transmission may be reduced relative to metals.

By choosing appropriate species, the density of wood may be matched to particular applications. The fibrous nature of wood permits internal absorption, retention and slow release of contactants; this may be called or is like the sherrybarrel effect. Used in wooden process plant, the danger of metallic contamination of product is avoided.

Simple size grading of pebbles so that the fluid always approaches the larger interstices formed by the larger pebbles may be arranged by using a wood of slightly lower density than the fluid for downward filtration, or by using wood of a slightly higher density than the fluid for upward filtration.

The grading effect may be achieved by using pebbles of all the same density.

The effect of gravity or buoyancy varies as the volume of any one pebble. The effect of fluid drag (viscosity) varies as the projected area of the pebble. Thus in downward filtration, it is possible to achieve flow conditions in which the larger pebbles move upwards because the effect of buoyancy is greater than the downward drag. At the same condition, smaller pebbles move downward because the downward drag exceeds the buoyancy. The reverse condition may be applied to upward flow.

Small pebbles of high density and large pebbles of low density (or vice versa) can be used to enhance the effect.

Up flow filtration against the underside of wooden pebbles allows the filter medium - the pebbles - to float in the fluids, without underside support. This allows both the pebbles and the retention to be gently removed from the substrate by lowering the fluid level, which should be particularly useful for residues of large area and low density (carrot peelings, papers, etc.).

Claims

1. A filtration, mass exchange or heat exchange apparatus comprising a tank, a plurality of buoyant or floatable members within the tank and means for passing fluid through the tank.

2. An apparatus according to claim 1, wherein the buoyant or floatable members take the form of wooden pebbles.

3. An apparatus according to claim 2, wherein the wooden pebbles are balls or ovoids.

4. An apparatus according to claim 3, wherein the buoyant or floatable members comprise a mass of larger pebbles and a mass of smaller pebbles.

5. An apparatus according to claim 4, wherein the largest of the larger pebbles has a radius of 2cm and the smallest of the smaller pebbles has a radius of 1cm.

6. An apparatus according to claim 2, wherein the wooden pebbles are toroids, dumbbell-shaped or banana-shaped.

7. A filtration apparatus according to any of the preceding claims, wherein the tank is provided with a fluid inlet at or adjacent to its top and with a fluid outlet at or adjacent to its bottom whereby fluid is arranged to flow through the tank in a downwards direction.

8. A filtration apparatus according to claim 7, wherein the tank is provided with a mesh screen or like support for supporting the buoyant or floatable members in the tank and for preventing said members from entering the outlet.

9. A filtration apparatus according to any one of claims 1 to 6, wherein the tank is provided with a fluid inlet at or adjacent to its bottom and with a fluid outlet at or adjacent to its top whereby fluid is arranged to flow through the tank in an upwards direction.

10. A filtration apparatus according to claim 9, wherein the tank is provided with a mesh screen or the like for preventing the buoyant or floatable members from entering the outlet.

11. A filtration, mass exchange or heat exchange apparatus substantially as described herein with reference to the drawings.

12. A filtration, mass exchange or heat exchange process comprising passing fluid through a mass of buoyant or floatable members.

13. A process according to claim 12, wherein the buoyant or floatable members take the form of wooden pebbles.

14. A filtration, mass exchange or heat exchange process substantially as described herein with reference to the drawings.

14. A process according to claim 12, wherein the buoyant or floatable members comprise the pebbles claimed in any one of claims 2 to 6.

15. A filtration, mass exchange or heat exchange process utilising the apparatus claimed in any one of claims 1 to 11.

16. A filtration, mass exchange or heat exchange process substantially as described herein with reference to the drawings.

Amendments to the claims have been filed as follows CLAIMS 1. A filtration, mass exchange or heat exchange apparatus comprising a tank, a plurality of buoyant or floatable members within the tank and means for passing fluid through the tank, wherein the buoyant or floatable members take the form of wooden pebbles.

2. An apparatus according to claim 1, wherein the wooden pebbles are balls or ovoids.

3. An apparatus according to claim 2, wherein the buoyant or floatable members comprise a mass of larger pebbles and a mass of smaller pebbles.

4. An apparatus according to claim 3, wherein the largest of the larger pebbles has a radius of 2cm and the smallest of the smaller pebbles has a radius of 1cam.

5. An apparatus according to claim 1, wherein the wooden pebbles are toroids, dumbbell-shaped or banana-shaped.

6. A filtration apparatus according to any of the preceding claims, wherein the tank is provided with a fluid inlet at or adjacent to its top and with a fluid outlet at or adjacent to its bottom whereby fluid is arranged to flow through the tank in a downwards direction.

7. A filtration apparatus according to claim 6, wherein the tank is provided with a mesh screen or like support for supporting the buoyant or floatable members in the tank and for preventing said members from entering the outlet.

8. A filtration apparatus according to any one of claims 1 to 5, wherein the tank is provided with a fluid inlet at or adjacent to its bottom and with a fluid outlet at or adjacent to its top whereby fluid is arranged to flow through the tank in an upwards direction.

9. A filtration apparatus according to claim 8, wherein the tank is provided with a mesh screen or the like for preventing the buoyant or floatable members from entering the outlet.

10. A filtration, mass exchange or heat exchange apparatus substantially as described herein with reference to the drawings.

11. A filtration, mass exchange or heat exchange process comprising passing fluid through a mass of buoyant or floatable members in the form of wooden pebbles.

12. A process according to claim 11, wherein the buoyant or floatable members comprise the pebbles claimed in any one of claims 2 to 5.

13. A filtration, mass exchange or heat exchange process utilising the apparatus claimed in any one of claims 1 to 10.