Improvements in or relating to baffle plates

Abstract

A baffle plate 100 for a gas / liquid mass transfer apparatus comprising a substantially channel shaped body comprising a substantially planar base 102, which defines a liquid support surface, with oppo

Description

The present invention relates to an improved baffle plate for use in a gas / liquid and vapour / liquid mass transfer apparatus and a mass transfer apparatus incorporating a plurality of such plates. Additionally, plates according to the present invention may also be utilised for effecting heat transfer between fluids for the purposes of evaporation and / or condensation, including for distillation. A gas / liquid mass transfer apparatus may be utilised to remove contaminants from a gas stream in a process typically referred to as “scrubbing". Such an apparatus operates on the well known principle of passing a gas through or over a liquid under conditions permitting very intimate contact between the gas and liquid such that a contaminant present in the gas is absorbed by the liquid. The contact between the liquid and the gas typically occurs in a column or stack through which the gas and liquid flow in opposite directions. The liquid is typically admitted into an upper portion of the stack whereupon it is caused to move downwardly through the stack under the influence of gravity, while the gas is admitted to a lower portion of the stack and is urged to flow upwardly therethrough. The stack includes a plurality of baffles which define a tortuous path through which the gas and liquid must flow. The purpose of the baffles is to create local zones of mutual high turbulence within the stack and to maximise the time during which the gas and liquid are in intimate contact with one another. The surface area over which the gas contacts the liquid is also important. It will be appreciated that the contaminants must pass through the surface and hence the larger the surface area the better the rate of transfer. Mass transfer apparatus of the type described above is particularly well adapted to the removal of such contaminants as hydrocarbons and solvents from a gas stream. The apparatus may advantageously be incorporated into an exhaust arrangement from, for example, a building or industrial apparatus, so as to clean gas exhausted therethrough. Contaminated gas, for example air, entering the mass transfer apparatus is cleansed of contaminants to such a degree that it may be exhausted to atmosphere without contravening any emissions regulations which may be applicable, while the liquid, for example water, may be recirculated through the apparatus before being processed to recover the contaminants accumulated therein. An example of a prior art baffle plate may be found in, for example, prior published patent application GB 2422561A. According to a first aspect of the present invention there is provided a baffle plate for a gas / liquid or vapour / liquid mass transfer apparatus, the baffle plate comprising a substantially channel shaped body having a substantially planar base with opposed substantially planar walls extending therefrom, wherein the base of the channel defines a liquid support surface, said liquid support surface being on the opposite side of the base from which the walls extend. Each wall may extend at least partially along the length of the base. In such an embodiment each wall may extend fully along the length of the base. Each wall may be of uniform height along its length. Both walls may be of substantially equal height. In an alternative embodiment each wall may be of a different height. The baffle plate may be of unitary construction. In such an embodiment the baffle plate may be formed from a single sheet of formable material. The baffle plate may, for example be folded from a sheet of metal such as aluminium alloy. In an alternative embodiment the baffle plate may be of multi-part construction. For example, the baffle plate may comprise a combined base and wall section, and a further wall section affixed to the combined base and wall section. In an alternative embodiment the baffle plate may be formed from a base section and two wall sections. According to a further aspect of the present there is provided a gas / liquid or vapour / liquid mass transfer apparatus comprising a housing having a plurality of superposed baffle plates as claimed in any preceding claim, said baffle plates being arranged to define a plurality of fluid flow paths through the housing, the housing being provided in an upper portion thereof with a liquid inlet and a gas outlet, and in a lower portion thereof with a gas inlet and a liquid outlet, the baffle plates being positioned within the housing such that a wall of an upper baffle plate is positioned over the liquid support surface of a lower baffle plate to define a restriction in a fluid flow path. The walls of two upper baffle plates may be positioned over the liquid support surface of a single lower baffle plate to define a restriction in a fluid flow path. An embodiment of the present invention will now be provided with reference to the accompanying drawings in which; Figure 1 shows a cross-sectionai side view of a gas / liquld mass transfer column, wherein the upper portion of the coiumn shows prior art type baffle plates, and the lower portion of the column shows baffle plates according to the present invention; Figure 2 shows the cross sectional side view of figure 1 with the position of liquid on the baffle plates, in use; and Figure 3 shows a perspective view of the baffle plates according to the present invention shown in the lower portions of figures 1 and 2. Referring firstly to the upper portions of figures 1 and 2 there is shown a side view of a gas / liquld mass transfer apparatus, generally designated 34, which includes a plurality of baffle plates 10 of the type described within GB2422561. The apparatus 34 comprises a hollow stack 36 having a rectangular cross section and within which there are provided a plurality of baffle plates 10. The stack 36 is provided in a lower portion 38 thereof a gas inlet 40 and a liquid outlet 42, and in an upper portion 44 thereof a liquid inlet 46 and a gas outlet 48. Between the respective lower and upper portions 38,44 of the stack 36 the baffle plates 10 extend in an overlapping arrangement alternatively from opposing sides 50,52 of the stack 36 to define a cascade for liquid passing from the liquid inlet 46 to the liquid outlet 42 and a sinuous path for gas passing from the gas inlet 40 to the gas outlet 48. It will be understood that the plates 10 extend fully across the stack 36 such that the respective sides thereof meet the corresponding walls to the stack 36 and are sealed thereagainst so as to prevent a gas or liquid flow path being provided around the sides of the plates 10. Similarly the edges 19 of the tail portions 18 of each plate 10 distal to the body to tail portion interface are sealed to the respective sides 50,52 of the stack 36 to prevent a gas or liquid flow path existing around the edges 19 of the plate tail portions 18. The alternating arrangement of the plates 10 from opposing sides 50,52 of the stack 36 ensures that the body portions 16 of adjacent plates 10 overlap one another in the centre of the stack 36. Looking at two adjacent overlapping plates identified as upper plate 10a and lower plate 10b it will be appreciated that the lower surface of the body portion 16a of the upper plate 10a overlaps the upper surface of the body portion 16b of the lower plate 10b. A heel portion 22a of the upper plate 10a is aligned with a lip portion 20b of the lower plate 10b, while the lip portion 20a of the upper plate 10a is aligned with the tail to body portion interface of the lower plate 10b. The heel and lip portions 22a,20b of the upper and lower plates 10af 10b co-operate to define a restriction or narrowing 54 between the plates 10a, 10b. In the embodiment shown the distance by which the heel portion 22a projects from the upper plate 10a is approximately two fifths of the distance between the overlapping body portions 16a, 16b of the plates 10a,10b. In use, the flow of gas through the apparatus 34 is indicated by arrows 56 on figure 2. Gas enters the stack 36 via the inlet 40 before passing through the restriction 54 defined between the lower 10b and upper 10a plates. The gas then passes between the overlapping body portions 16a, 16b of the plates 10a, 10b before entering a chamber 58 defined overlapping tail portions 18, 18a of the plates. The gas exits the chamber via the restriction 54 defined between overlapping plates. It will be appreciated that the in moving to and from the space defined by overlapping body portions 16 of successive plates 10 the gas is caused to turn through substantially 180 degrees. The flow of liquid through the apparatus 34 is indicated by arrows 60. The liquid enters the stack 36 via the Inlet 46 which is positioned above the tali portion 18a of the uppermost plate in the stack 36. Upon contacting the tail portion 18a the liquid is caused to run towards and over the body portion 16a by virtue of the inclination of the tail portion 18a relative to the body portion 16a. Upon reaching the lip portion 20a of the plate 10a the liquid cascades onto the tail portion 18b of the second uppermost plate 10b. The curved nature of the lip portion 20a assists in the smooth flow of the liquid from the uppermost plate 10a to the second uppermost plate 10b. The liquid continues to cascade from plate to plate until it reaches the outlet 42 in the lower portion of the stack 36. The interaction between the respective gas and liquid flows through the stack 36 is illustrated between plates is shown in figure 2. The gas flow 56 through the restriction 54 defined between the plates 10a,10b causes a backing up liquid on the plates 10a, 10b with a local increase in hydraulic depth and a corresponding reduction in liquid flow rate through the stack 36. The gas flow through the restrictions 54 defined between the overlapping plates 10 thus reduces the flow rate of liquid through the apparatus 36 compared to the flowrate which would be expected should the gas flow not be present. It will thus be appreciated that the time period during which liquid is retained within the apparatus and thus exposed to intimate contact with the gas is increased with a consequent increase in the efficiency of mass transfer of contaminants present in the gas to the liquid. Also, the increased gas velocity increases turbulence (also referred to as turbulent roiling) in both the gas and the liquid providing a further improved mass transfer. It will also be appreciated that the inclined nature of the tail portions 18 of the plates 10 serves to urge the backed up liquid towards the lip portions 20, 20a, 20b of the plates 10 with the result that liquid flow through the apparatus 36 is not stalled by the gas flow. As will be observed in the upper part of figure 2, the backing up of the liquid produces a standing wave with a peak in the vicinity of the lip portions 20, 20a, 20b. These peaks define the mass transfer locations and thus there exists a single mass transfer location per plate 10. Testing, in line with section 14-68 of Perry's Chemical Engineers Handbook (7th Edition) has demonstrated a high mass transfer coefficient for the arrangement described in GB 24226561. There is thus a defined relationship between improving mass transfer and increasing gas pressure drop across the plates. Referring now to the lower portions of figures 1 and 2 and figure 3, there are shown baffle plates, generally designated 100, which correspond to an embodiment of the present invention and which seek to improve mass transfer compared to the arrangement of GB24226561. More specifically, the present invention seeks to increase the area of the vertical gas flow paths within the stack so as to reduce back pressure. Additionally, the present invention seeks to increase the area of the active mass transfer sites when compared to a stack of similar cross-sectional dimensions. Each baffle plate 100 incudes a elongate planar central portion or base 102 that is substantially rectangular when viewed in plan, and opposed elongate planar edge portions or walls 104, 106 which extend downwardly from each longer side of the base 102. Each baffle plate 100 is thus in the form of a substantially U-shaped channel. Each baffle plate 100 extends fully across the hollow stack 36 between opposing sides thereof. This is in contrast the previously descried baffle plates 10 which as described above extend In an overlapping arrangement alternatively from opposing sides 50,52 of the stack 36 Each baffle plate 100 may be formed from a sheet of material, for example a sheet of a metallic material. More specifically, and by way of example only, each baffle plate 100 may be formed from a sheet of stainless steel that is folded so as to produce the U-shape of the baffle plate 100. In such an embodiment the stainless steel may have a thickness of approximately 1mm. It will be appreciated that other materials for the baffle plate 100 may be used including, but not limited to, other metals and metal alloys, plastics materials, ceramics, glass and wood. It will further be appreciated that the baffle plates 100 may be formed by operations other than a folding operation. For example, and depending upon the material chosen each baffle plate may be formed from one or more of a pressing, casting, moulding, machining and additive manufacturing process. The baffle plates 100 may, for example, be formed unitarily by another operation, for example by extrusion. Alternatively the baffle plates 100 may be of a multi piece construction. For example each baffle plate 100 may be formed from two or more U-shaped channel sections that are joined together. In an alternative embodiment each baffle plate 100 may be of a two part construction wherein one part comprises the base 102 and one of the walls 104,106 and the other part comprises the other of the walls 104,106, In yet another alternative embodiment each baffle plate 100 may be of a three part construction comprising the base 102 and each wail 104, 106. In the embodiment shown the walls 104, 106 are each of equal dimensions. In an alternative embodiment each wall 104, 106 may be of differing dimensions such that one wall 104,106 extends a different distance from the base than the other wall 104, 106. Referring again to the figures, the baffle plates 100 are provided within the stack 36 in three rows, namely an upper row 108, a middle row 110 and a lower row 112. The provision of three rows 108,110 and 112 is shown by way of illustrative example, and the stack 36, in use, may be provided with a greater number of baffle plate rows. In the embodiment illustrated each row is comprised of two baffle plates 100. Each row is provided with a baffle plate 100 having an edge portion 104, 106 that lies against one of the sides 50, 52 of the stack 36. The other of the baffle plates 100 of a row is positioned such that it is spaced from the other of the sides 50,52 of the stack. The baffle plate 100 of a row are spaced apart from one another such that a first passage 114 is defined between both baffle plates 100, and a second passage 116 is defined between one of the baffle plates 100 and a side of the stack 50,52. As can be observed in the figures, the baffle plates 100 of upper and lower rows 108,112 are provided in vertical registersuch that the baffle plates 100 and passages 114,116 align with one another. The baffle plates 100 of the middle row 110 are not provided in vertical register with the baffle plates of the upper and lower rows 108,112. Instead, the baffle plates 100 of the middle row 110 are positioned such that the central portion 102 of each baffle plate 100 lies below one of the passages 114.,116 of the upper row 108. Similarly, the passages 114,116 of the middle row 110 are positioned above central portions 102 of the lower row baffle plate 100 central portions 102. It will further be noted that, with the exception of the baffle plate edge portions 104 that lie against the sides of the stack 36, the baffle edge portions 104 of an upper row overlie the baffle central portion 102 of a lower row. As such a flow restriction 120 is defined between adjacent baffle rows. It will be further appreciated that the alignment of the baffle plate rows 108,110,112 defines a sinuous gas flow path through the stack 36 between gas inlet 40 and the gas outlet 48 as shown by arrows 122. The stack 36 of the present invention provides multiple sinuous gas flow paths as opposed to a single gas flow path. The gas flow through the restrictions 120 defined between the overlapping plates 100 thus reduces the flow rate of liquid through the stack 36 compared to the flow rate which would be expected should the gas flow not be present. It will thus be appreciated that the time period during which liquid is retained within the apparatus and thus exposed to intimate contact with the gas is increased with a consequent increase in the efficiency of mass transfer of contaminants present in the gas to the liquid. As is shown in figure 2 the accelerated gas flow through the restrictions 120 also causes liquid 124 to be retained upon the baffle plate central portions 102. This promotes mixing at the gas to liquid interface and thus improves scrubbing efficiency. It will be noted that retained liquid 124 is formed into standing waves peaks by the gas flow. As noted above, these peaks define the mass transfer locations within the stack 36. On the baffle plates 100 located adjacent the sides 50,52 of the stack 36 there is defined a single peak. On the baffle plates 100 that are spaced from the sides 50,52 of the stack 36 the retained liquid forms two peaks. Thuss for each row 108, 110, 112 of baffle plates 100 there are provided three liquid peaks and three corresponding mass transfer locations. This is in contrast to the prior art arrangement where in there is a single liquid peak / mass transfer location per row. The induced hydraulic depth, arising from the counter current gas flow, is sufficient to impel liquid flow over the horizonal surfaces of the baffle plates, without the need for any part of the plate to slope. The following table sets out data measured in connection with the testing of a stack having a plan area of 0.0242 m2 (110mm x 220mm) fitted with prior art baffle plates of the type described in GB2422561, Mass transfer coefficient is expressed in 5 Kg.molesCO2 per hour per m3 packing per atm. partial pressure of CO2. The gas flow rate through the stack is 63m3 / hr. Spacing (mm) Number of plates Liquid flow (l / mln) Mass Transfer Coefficient Pressure drop (mB) Liquid depth (mm) Dwell time (seconds) 25 25 2 90 7.5 3.4 2.0 25 25 4 100 10.7 5.3 1.6 25 25 6 112 27.0 8.9 1.8 30 20 4 68 6,2 6.2 1.8 30 20 8 78 8.7 8.3 1.2 30 20 12 84 11.2 9.7 1.0 In testing a stack with the mostly the same parameters except for a slightly enlarged 10 plan area of 0.0315 m2, baffle plates according to the present invention and a gas flow rate through the stack of 82m3 / hrJ the following observations were made. Liquid flow (l.min) Pressure drop (mB) 2 3,0 4 3.0 6 3.2 8 3.2 10 3.3 12 3.3

Claims

1. A baffle plate for a gas / liquid or vapour / liquid mass transfer apparatus, the baffle plate comprising a substantially channel shaped body having a substantially planar base with opposed substantially planar walls extending therefrom, wherein the base of the channel defines a liquid support surface, said liquid support surface being on the opposite side of the base from which the walls extend.

2. A baffle plate as claimed in claim 1 wherein each wall extends at least partially along the length of the base.

3. A baffle plate as claimed in claim 2 wherein each wall extends fully along the length of the base.

4. A baffle plate as claimed in any preceding claim where each wall is of uniform height along its length.

5. A baffle plate as claimed in claim 4 wherein both walls are of substantially equal height.

6. A baffle plate as claimed in any of claims 1 to 4 wherein each wall is of a different height.

7. A baffle plate as claimed in any preceding claim wherein the baffle plate is of unitary construction.

8. A baffle plate as claimed in claim 7 wherein the baffle plate is folded from a single sheet of formable material.

9. A baffle plate as claimed in any of claims 1 to 6 wherein the baffle plate is of multi-part construction.

10. A baffle plate as claimed in claim 9 wherein the baffle plate comprises a combined base and wall section, and a further wall section affixed to the combined base and wall section.

11. A baffle plate as claimed in claim 9 wherein the baffle plate is formed from a base section and two wall sections.

12. A gas / liquid or vapour / liquid mass transfer apparatus comprising a housing 5 having a plurality of superposed baffle plates as claimed in any preceding claim, said baffle plates being arranged to define a plurality of fluid flow paths through the housing, the housing being provided in an upper portion thereof with a liquid inlet and a gas outlet, and in a lower portion thereof with a gas inlet and a liquid outlet, the baffle plates being positioned within the housing such that a wall of an upper baffle plate is 10 positioned over the liquid support surface of a lower baffle plate to define a restriction in a fluid flow path.

13. A mass transfer apparatus as claimed in claim 12 wherein walls of two upper baffle plates are positioned over the liquid support surface of a single lower baffle plate. 15A

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