WO1998050752A1

WO1998050752A1 - Structured packing for mass transfer and/or heat exchange between a liquid and a gas

Info

Publication number: WO1998050752A1
Application number: PCT/NL1998/000247
Authority: WO
Inventors: Geert Feye Woerlee
Original assignee: Sulzer Chemtech Ag
Priority date: 1997-05-06
Filing date: 1998-05-05
Publication date: 1998-11-12
Also published as: BR9809220A; NL1005990C2; CN1255194A; EP0980505A1; AU7456898A; JP2001523163A

Abstract

The invention relates to a structured packing (1) for mass transfer or heat exchange between a liquid and a gas. The structured packing comprises a multiplicity of sheets (2, 3) with corrugations (G) parallel to one another. Adjoining sheets are arranged crosswise with respect to the direction of their channels. The corrugations of each sheet have a corrugation height (H) and a corrugation width (B). The ratio of the corrugation height (H) to corrugation width (B) satisfies the equation H/B ≥ 0.75, preferably satisfies the equation H/B ≥ 1 and even more preferentially satisfies the equation H/B ≥ 2. The sheets with corrugations can have a triangular or sine-wave-shaped corrugation profile. The direction of the channels in the sheets can be arranged at an angle of approximately 45° to 70° with respect to the vertical.

Description

STRUCTURED PACKING FOR MASS TRANSFER AND/OR HEAT EXCHANGE BETWEEN A LIQUID AND A GAS

The present invention relates to a structured packing for mass transfer and/or heat exchange between a liquid and a gas, wherein the structured packing comprises a multiplicity of sheets with corrugations parallel to one another, the corrugations in the sheets delimiting channels, and wherein adjoining sheets with corrugations are arranged with their channel directions crosswise, and wherein the corrugations of each sheet have a corrugation height H and a corrugation width B.

Structured packings of this type are generally know per se. In the case of sheets having sine-wave-shaped corrugations, in general a value of around the number π is taken for the corrugation width whilst a figure of around a value of 1 is taken for the corrugation height. In this context, the unit of π and the value 1 is in general in cm. With a structured packing of this type adjoining sheets with corrugations (also termed corrugated sheets) are always arranged crosswise with respect to one another, such that the flow channels delimited by the corrugated sheets cross one another, so-called boundary surfaces being formed at the location where the open sides of two crossing channels cross one another. Gas streams flowing in different directions through channels which cross one another come into contact with one another at said boundary surfaces. The pressure drop over a packing as a consequence of flow resistances to which gas streams flowing therethrough are subjected is to a large extent dependent on, on the one hand, the flow resistance to which the gas streams are subjected at the contact surface with the sheets with corrugations and, on the other hand, on the resistance as a consequence of flow effects produced at the boundary surfaces, which are also termed boundary surface effects, such as turbulence. The view held in this context is that the gas phase does not mix well or mixes less well if the ^H/_B ratio is too high. For this reason, a ratio of corrugation height to corrugation width which is less than 0.3 to 0.5, that is to say ^H/_B ≤ 0.3 - 0.5, is used in practice. In order to keep the pressure drop over such a structured packing relatively small, the sheets with corrugations, which in practice are arranged vertically, are arranged with the direction of their channels usually at an angle of approximately 30° with respect to the vertical, which angle will, however, not exceed 45° in practice. The reason for this is that the flow resistance over the packing becomes too great when the angle of the channel direction with respect to the vertical is (relatively) large.

The aim of the present invention is to provide an improved structured packing of the type indicated in the preamble.

Said aim is achieved according to the invention in that the ratio of corrugation height H to corrugation width B satisfies the equation ^H/_B ≥ 0.75. As the ^H/_B ratio increases, the influence of effects giving rise to flow resistance at the so- called boundary surfaces decreases because the average distance of the gas from said boundary surface is increased at these locations with a larger ^H/_B ratio. However, this reduction in the flow resistance as a consequence of so-called boundary surface effects is offset by an increase in the flow resistance experienced at the contact surface between the corrugated sheets arid the gas streams since the contact surface per channel between the sheet and the gas stream becomes larger when the ^H/_B ratio is increased. Surprisingly, however, it has been found that the increase in the flow resistances at the contact surfaces is outweighed by the advantages which are achieved as a result of the reduction in the size of the boundary surfaces (which, incidentally, is also accompanied by a reduction in the total number of boundary surfaces in a packing) and that other expected disadvantages also do not arise or barely arise or are outweighed by the advantages achieved. The surprising, advantageous effect is found to be already apparent at ^H/_B ratios of ≥ 0.75, starts to manifest itself clearly at ^H/_B ratios of ≥ 1 and is particularly apparent at an ^H/_B ratio of ≥ 2 . It will be clear that, depending on the type of corrugation profile, in practice an upper limit will also be imposed on the ^H/_B ratio since if the ^H/_B values are too high the flow surface will assume a slit-like shape to too great an extent, with the associated disadvantages of turbulence effects and/or flow resistance effects. Furthermore, in particular in the case of non-rectangular, for example cylindrical, containers, there will be a minimum number of sheets needed in order to achieve an adequate degree of filling of the container.

Especially in the case of metal sheets with corrugations, it will be advantageous according to the invention, on the grounds of cost, if said sheets have a triangular, trapezium- shaped or rectangular corrugation profile. This is because profiles of this type, in particular triangular corrugation profiles, can be produced relatively easily from sheet metal or suitable plastic sheeting by folding. However, on flow technology grounds, in practice preference will frequently be given according to the invention to a sheet with corrugations which have an approximately sine-wave-shaped, preferably pure sine-wave-shaped, corrugation profile.

An important parameter when designing structured packings of the type mentioned in the preamble, and also in the case of structured packings according to the invention, is the so- called specific surface area, which has m²/m³ as its unit. Said specific surface area is in fact the contact surface of the sheets per cubic metre packing. Values widely used in practice for this so-called specific surface area are: approximately 125 πT¹ , approximately 200 πf¹ , approximately 250 πf¹ and approximately 500 rrf¹. Assuming that a certain specific surface area is required, desirable or prescribed, B and H will, according to the invention, advantageously form a solution to the equation:

*> - iϊX sm^α + — πH) da

which can be solved numerically. It follows from this equation that, for a certain specific surface area, there is a relationship between B and H which can be represented as a curve in a two-dimensional plot. In order to achieve optimum liquid flow over the structured packing it is advantageous according to the invention if the sheets with corrugations are arranged essentially vertically. According to an advantageous embodiment of the invention, one or more of the corrugated sheets are arranged with their channel direction at an angle of at least approximately 45° with respect to the vertical. Preferably, one or more of the corrugated sheets are arranged with their channel direction at an angle of approximately 55° to 65° with respect to the vertical, such as at an angle of approximately 60° with respect to the vertical. Such angles for the channel direction are actually not achievable with structured packings which consist of sheets with corrugations according to the prior art since the flow resistance of the packing then assumes too high a value. However, with relatively larger ^H/_B ratios in accordance with the invention it is, surprisingly, fou:ιd to be possible to make the angle of the channel direction with respect to the vertical larger, even up to approximately 70°. The advantage of a larger angle of the channel direction with respect to the vertical is that better mass transfer between the gas flowing upwards via the structured packing and the liquid flowing downwards over the structured packing is achieved by this means. It must be pointed out that, compared with the prior art, according to the invention highly advantageous effects are also achieved with channel directions of less than 45° with respect to the vertical, such as at 10-45° with respect to the vertical. With such smaller angles, the pressure drop over the packing according to the invention is lower than is the case with conventional packing. In order to restrict the flow resistance of the gas in the edge regions adjoining the wall or the wall section at the transition from one channel to another channel when the structured packing is arranged in a container in such a way that the corrugated sheets delimit channels extending between opposing walls or wall sections thereof, especially when the channels are at relatively large angles with respect to the vertical, it is advantageous according to the invention if the sheets with corrugations are provided with holes at the channel ends adjoining the wall sections. Such holes make it easier for the flowing gas to pass from a channel running in one direction into a channel running in another, crosswise direction. Especially for containers having a relatively large cross- sectional area, or a relatively large diameter in the case of cylindrical containers, such an edge region will preferably extend up to 10-20 cm from the wall, depending on the so-called specific surface area. In this context relatively large cylindrical containers are understood to be containers having a diameter of 1 or more. In the case of relatively small containers, in particular, such an edge region will extend over a distance of approximately 10 % to 20 % of the width/depth of the container and in the case of a cylindrical container over a distance of approximately 10 % to 20 % of the diameter of said container. According to an advantageous embodiment, the holes will have a diameter of at least 2 mm up to, preferably, at most 20 mm, or a flow area equivalent to this in the case of non-circular holes.

The invention will now be explained in more detail with reference to the drawing. In the drawing:

Figure 1 shows, diagrammatically , a triangular corrugation profile, in which a few parameters thereof have been indicated in more detail;

Figure 2 shows a perspective and diagrammatic view of part of a structured packing according to the invention made up of four layers;

Figure 3 shows a diagrammatic view, partially in cross- section, of a container according to the invention;

Figure 4 shows diagrammatically, one corrugation with a sine-wave-shaped corrugation profile;

Figure 5 shows a plot of an ^H/_B relationship for a sine- wave-shaped corrugation profile; and

Figure 6 shows a highly diagrammatic representation of a test set-up. Figure 1 shows, diagrammatically, a triangular corrugation profile having a corrugation height H and a corrugation width B. It will be clear that said corrugation height H and corrugation width B are assignable in an identical manner to the corrugations of the sine-wave-shaped corrugation profile shown in Figure 4.

Figure 2 shows, by way of example, in highly diagrammatic form, a structured packing according to the invention consisting of, by way of example, four layers of sheets with corrugations (also termed corrugated sheets), each having a triangular corrugation profile. With reference to sheet 2 with corrugations it will be clear that each sheet 2 with corrugations delimits a multiplicity of channels extending transversely to the direction of corrugation G. Said channels 6, which are open on the underside, or channels 7, which are open at the top, thus have a channel direction K extending transversely to the direction of corrugation G. It will be clear that in this example according to Figure 2, the corrugated sheets 3 are identical to the sheets with corrugations 2 except for their orientation. However, as wiJ 1 be clear to an average person skilled in the art, it is in no way necessary, as far as the essence of the invention is concerned, that the sheets with corrugations 2 and 3 are identical except for their orientation nor that the sheets with corrugations in the layers 2 are identical to one another. As far as the essence of the invention is concerned, it is also not important that G and K are perpendicular to one another. As will also be apparent from Figure 2, where channels 7, open towards the top, and channels 6, open towards the bottom, cross, imaginary boundary surfaces 4 delimiting said channels at this location are formed. Furthermore, it will be apparent that when gas flows through the channels of adjacent sheets, boundary surface effects will arise at the location of said boundary surfaces 4, which effects have an adverse influence on the flow resistance of the packing as a whole. Furthermore, gas flowing through the packing will be subjected to further flow resistance on coming into contact with the sheets with corrugations themselves . According to the invention it has been found that when a value of ≥ 0.75 is taken for the ratio of corrugation height H to corrugation width B there is an improvement in the flow resistance over the packing as a whole. Especially with ^H/_B ratios of ≥ 1 and above this is so obvious that major advantages are achieved, which advantages can be even further increased if the ^H/_B ratio is ≥ 2 .

The embodiment according to Figure 3 shows a container according to the invention which is provided with a packing 1 according to the invention.

With a container of this type, liquid is distributed over the packing via a spray device 10 at the top. Said liquid flows downwards over the surfaces of the sheets with corrugations in a manner such that a liquid film is formed on the surfaces of the sheets. A gas is supplied at the bottom of the packing, which gas flows upwards through the packing in counter-current to the liquid (film). Transfer between the liquid and the gas can take place during the counter-current flow. It is pointed out that the flow of gas and liquid through the packing can optionally also take place in co-current or by transverse flow. With packings according to the invention advantages are also obtained with co-current and transverse current flow.

With this set-up, the corrugated sheets of packing 1 are arranged vertically with a channel direction K at an angle of essentially greater than 50° with respect to the vertical (however, angles of less than 50° are also usable and offer significant advantages according to the invention) . A lower packing 11 is also arranged beneath the upper packing 1. In the case of the upper packing 1 , the channel direction runs at an angle of approximately 60° with respect to the vertical. In the case of the lower packing 11 , the channel direction runs at an angle of approximately 45° with respect to the vertical, which angle under certain circumstances can also be smaller than 45° with respect to the vertical, such as, for example, 30°. So- called "flooding" problems are counteracted by making the angle of the channel direction with respect to the vertical smaller in the lower packing 11. Flooding in this context is understood to mean that a droplet remains suspended from the bottom of the packing, which droplet is easily entrained by the gas flowing upwards. This phenomenon of droplets remaining suspended is counteracted by positioning the channels of the lower section of the packing at a steeper angle. It is pointed out that said positioning of the channels in the lower section of the packing at a steeper angle in order to counteract flooding can also be employed independently of a relatively larger ^H/_B ratio and also independently of relatively large angles for the channel direction of the upper packing 1. If, however, as is advantageously possible according to the invention, relatively large angles are used for the direction of the channel with respect to the vertical, in particular angles in a range of 50° to approximately 70°, the so-called flooding problem at the bottom of the packing is then exacerbated.

Figure 3 further illustrates what is understood by an edge region adjoining the wall or the wall section of the container. In this figure said edge region is indicated by R and the magnitude of R will in general be approximately 10-20 cm for containers having a diameter of 1 m and above, whereas for containers having a smaller diameter said R will be approximately 10 % to 20 % of the diameter. The corrugated sheets are provided with holes 5 in said so-called edge region, which holes make it easier for the gas to pass from one channel sloping in one direction to a crossing channel, sloping in the other direction, in the edge regions, in order thus to reduce the flow resistance of the packing. It will be advantageous to provide the edge regions of the packing with such holes 5 especially when, according to the invention, relatively large angles are used for the channel direction with respect to the vertical .

The way in which, starting from a certain specific surface area A , values for B and H can be determined in the case of a corrugated sheet having a sine-wave-shaped profile will be illustrated below with reference to Figures 4 and 5.

A sheet folded in a sine wave shape can be described as follows in terms of parameters: y = — (1 - cos α)

where a = 2π^, so that x = -^-B (1 )

B 2π The length of a small section of said sheet (see Figure 4) can then be described as:

Starting from equation ( 1 ) dy and dx can now be written as:

H B dy = — sin a da and dx = — da (3)

2 2π

Entering equation (3) in equation (2) then gives:

The integral of equation (4)

then gives the total length of the sheet, which is usable for determination of the specific surface area A in accordance with equation (6) given below:

2 s

A„ = f 2 sinα + — B \² dan (6) ^p BHz BB J J oo \ nH

The relationship from equation (6) can now be solved numerically for B as a function of H or vice versa when a value is taken for A P. Figure 5 shows the relationship between B and

H when A is taken as 250 m . This curve is indicated by F. The line H = 0.75B has also been drawn in Figure 5. Looking at the intersection of said line with the curve F it can be seen that said intersection is located a little above that point on the curve F which is closest to the origin. In this context the origin is understood to be the point H = 0 and B = 0.

With reference to equation (6) and Figure 5 it can also be pointed out that corresponding curves for other values of A_p can be produced in a simple manner on the basis of Figure 5. For instance, the curve for A = 500 m^"1 can be obtained simply by dividing all values for B and H in Figure 5 by 2. If A is taken as 100 m^"1 , all values for B and H from Figure 5 must then be multiplied by 2.5. The curves for further values of A can also be determined correspondingly, as will be obvious to a person skilled in the art. A number of experiments were carried out using a test setup as shown highly diagrammatically in Figure 6, in which experiments packings according to the invention were compared with packing from the prior art .

EXPERIMENT I

In a first experiment six packing units, numbered 21 and 22, were placed above one another in a cylindrical container having a diameter of 19.2 cm. In this experiment each packing unit 21, 22 consists of a number of vertically arranged sheets with corrugations parallel to one another. As is shown diagrammatically in Figure 6, the sheets with corrugations 21 and 22 of adjacent packing units are positioned perpendicular to one another. In the packing units the channel direction of each sheet is in all cases at an angle of 45° to the vertical. The corrugation profiles in this experiment are essentially triangular in shape, with a corrugation height H and a corrugation width B, as is indicated diagrammatically in Figure 1. In other respects use was made of two test set-ups, the specific structural characteristics of which are given in the table below:

TABLE I

As can be seen from Table I, the so-called specific surface area per packing was set at Ap = 250 m ¹ and set-up I was designed for an ^H/_B ratio of 0.49 and set-up II was designed for an ^H/π ratio of 1.13, for which the H and B can then therefore be read off from the plot in Figure 5. Air was fed through both set-ups I and II at a so-called superficial gas rate U of, successively, 1, 2, 3 and 5 m/s. Said air was fed from bottom to top through the packings arranged in a cylinder, in accordance with Figure 6. In this experiment the pressure drop over the column was then measured per set-up and superficial gas rate, in each case. In this context superfacial gas rate U (in m/s) is understood to be the average flow rate of the air with respect to the cross-sectional area of the cylinder, which cross-sectional area is therefore 289.53 cm² here. For this experiment, the air drawn in from the environment was at a temperature of 20°C and atmospheric pressure. The results of the measurements in this experiment are given in Table II below.

TABLE II

It can be seen from the experimental measurement data in Table II that the packing according to the invention (in set-up II) has a pressure drop which in general is approximately 25 % lower than the pressure drop in the case of set-up I.

EXPERIMENT II Experiment II was carried out using the same set-ups I and II as used for Experiment I. The difference in this case is that in Experiment II a water film was also introduced on to the packings by spraying water at the top of the set-up shown in Figure 6. The quantity of water W is expressed here in kg/s/m², that is to say in kg water per second distributed over a cross-sectional area of the cylindrical container, which in this case is approximately 289.5 cm², as has been indicated above. In Experiment II the superficial gas rate U at which a pressure drop per metre packing of 800 Pa/m is obtained was determined for three quantities of water W per set-up. The results obtained in this experiment are shown in Table III.

TABLE III

It can be seen from Table III that with set-up II 25 % higher gas rates are permissible before a pressure drop of 800 Pa/m is obtained. This means that with a packing according to the invention an approximately 25 % higher throughput capacity is achievable for a given pressure drop, which in this example is 800 Pa/m.

Claims

1. Structured packing for mass transfer and/or heat exchange between a liquid and a gas, wherein the structured packing comprises a multiplicity of sheets with corrugations parallel to one another, the corrugations in the sheets delimiting channels, and wherein adjoining sheets are arranged crosswise with respect to their channel direction, and wherein the corrugations of each sheet have a corrugation height H and a corrugation width B, characterised in that the ratio of corrugation height H to corrugation width B satisfies the equation ^H/_B ΓëÑ 0.75.

2. Structured packing according to Claim 1 , characterised in that the ratio of corrugation height H to corrugation width B satisfies the equation ^H/_B ΓëÑ 1.

3. Structured packing according to Claim 1 or Claim 2, characterised in that the ratio of corrugation height H to corrugation width B satisfies the equation ^H/_B ΓëÑ 2 .

4. Structured packing according to one or more of the preceding Claims 1-3, characterised in that one or more of the sheets with corrugations have a triangular corrugation profile.

5. Structured packing according to one or more of the preceding claims 1-3, characterised in that one or more of the sheets with corrugations have a sine-wave-shaped corrugation profile.

6. Structured packing according to one or more of the preceding claims, wherein the sheets with corrugations have a sine-wave-shaped corrugation profile, characterised in that for a certain, desired specific surface area A , B and H form a solution of the equation:

sirr╬▒ + B \² da

B Jo ^'N ╧Ç-ff)

which can be solved numerically.

7. Structured packing according to one or more of the preceding claims, characterised in that the sheets with corrugations are arranged essentially vertically.

8. Structured packing according to Claim 7, characterised in that one or more of the sheets with corrugations are arranged with their channel direction at an angle of approximately 45┬░ to 70┬░ with respect to the vertical.

9. Structured packing according to Claim 7 or Claim 8, characterised in that one or more of the sheets with corrugations are arranged with their channel direction at an angle of approximately 55┬░ to 65┬░ with respect to the vertical, for instance at an angle of approximately 60┬░ with respect to the vertical.

10. Container, in particular a container of an exchanger, provided with a structured packing according to one of the preceding claims, wherein the corrugated sheets delimit channels extending between opposing walls or wall sections of the container, characterised in that the sheets with corrugations are provided with holes at the channel ends adjoining the wall sections.

11. Container according to Claim 10, characterised in that the holes are made in an edge region which adjoins the wall or the wall section and extends up to 10 - 20 cm from the wall .

12. Container according to Claim 10 or Claim 11 , characterised in that the holes are made in an edge region which adjoins the wall or the wall section and extends over a distance of approximately 10 % to 20 % of the diameter of the container .

13. Container according to one or more of Claims 10 - 12, characterised in that the holes have a diameter of at least 2 mm up to, preferably, at most 20 mm or have a flow surface area equivalent to this.