GB1596738A

GB1596738A - Gas/liquid contacting

Info

Publication number: GB1596738A
Application number: GB49385/76A
Authority: GB
Original assignee: LINKROSE Ltd
Current assignee: LINKROSE Ltd
Priority date: 1976-11-26
Filing date: 1976-11-26
Publication date: 1981-08-26
Also published as: IT1117139B

Description

(54) GAS/LIQUID CONTACTING (71) We, LINKROSE LIMITED, a British Company of, 17 Chedworth Close, Selly Oak, Birmingham B29 4LS, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to gas/liquid contacting.

The terms "gas" and "liquid" are to be construed widely herein, both to include gaseous mixtures of gases and liquid mixtures of liquids and to include, in the case of gases, vapours and, in the case of liquids, solutions of gas and/or liquid and/or solid in a liquid and dispersions of liquid and/or solid in liquid.

The term "gas" also includes suspension of liquids or solids in a gas or vapour.

There is a wide variety of industrial applications where it is desired intimately to contact a gas and a liquid. One of the most important of these is he dissolution of gases in liquids but a wide variety of other gas/liquid contacting processes are used in industry for effecting physical, physico-chemical, chemical, fermentation or biological reactions, often accompanied by the transfer of heat.

Another important area of gas/liquid contacting is gas scrubbing and stripping, and a further area is deentrainment of solids suspended in a gas stream.

In the field of the dissolution of gases in liquids, numerous attempts have been made to provide simple, efficient and robust apparatus for effecting dissolution. Examples of applications of a gas dissolving are dissolving air or oxygen into water in e.g. effluent treatment processes and/or flotation processes. Additionally it is in many cases more satisfactory to carry out a reaction with a gas if that gas is first dissolved into a liquid carrier and the liquid carrier with the gas dissolved therein then fed into an absorber, catalytic reactor or like apparatus. Adsorption and ion exchange processes, including processes for liquid, solid contacting in many cases also require supply of liquid with gas dissolved therein.

A wide variety of apparatus has been pro posed for dissolving gases into liquids. For example British Patent Specifications 461,439, 684,782, 1,326,457 and 1,446,402 and United States Specifications 3,092,678, 3,476,366, 3,804,255 and 3,926,588 all disclose various apparatus for dissolving gases in liquids.

A problem with some known apparatus is loss of the gas it is desired to dissolve by the formation of bubbles which rise to the surface of the liquid and escape to the atmosphere or are carried out with the liquid stream or are vented from the device. Clearly this can be tolerated, though it is economically disadvantageous, where the gas is air or oxygen and cannot be tolerated in cases of high cost or reactive gases.

Gas loss may be prevented when the gas liquid ratio is below the solubility limits by carrying out the gas/liquid contacting in an upwardly closed column with liquid downflow but in such cases there is a tendency to formation of a gas filled space above a liquid surface. In order to preserve the column substantially full of liquid and dispersion, gas then needs to be vented, either continuously or at regular intervals from the gas filled space. The presence of such a space and the operation of venting seriously reduce the efficiency of the contacting process. Additionally, venting and recycling requires control apparatus which is often costly and liable to malfunction in use.

We have now found that if certain conditions are observed it is possible to carry out gas/ liquid contacting in a very wide variety of applications with considerable economy of energy input, high approach to equilibrium conditions, very high absorption and reaction efficiency, with substantially 100% gas utilisation and with relatively simple apparatus.

According to the present invention there is provided a method of contacting gas and liquid which comprises introducing the gas and liquid into a column, at least the liquid being introduced into the top of the column in the form of at least one stream, the velocity of which stream (s) is sufficient to generate and maintain at least in an upper region of the column, a substantially continuous foam formed of close-packed bubbles of the gas in the liquid extending across the entire crosssection of the column, driven to violent agita tion and backmixing by the incoming stream (s) of liquid, and of relatively uniform bubble size, the velocity of the inlet stream (s) of liquid and the rate of introduction of the gas being sufficient to prevent gas accumulating at the top of the column, and removing at least treated liquid from a lower region of the column.

By working in this way it is possible to obtain very stable operation accompanied by very high contact efficiency. The high velocity incoming stream of liquid causes very substantial turbulence and backmixing in the foam which it produces which extends from the top of the column down to an appropriate level therein. The gas may be introduced into the column together with the incoming stream of liquid or it may be introduced at any other point of the column. It is not necessary to take care to introduce the gas as a fine dispersion since even if the gas is introduced as a continuous stream, it is very rapidly broken up into the foam by the incoming stream of high velocity liquid.

There is indeed a fundamental difference in operation between the method of gas liquid contacting of the present invention and that employed in prior art apparatus. In the prior art apparatus, attempts are made to disperse the gas into a large number of fine bubbles, and to ensure, e.g. by appropriate flows, that the bubbles are maintained in the treatment chamber or vessel for a sufficient time to enable them to become substantially completely dissolved as they rise up in the relatively static liquid. In contrast to this, when dissolving a gas in a liquid according to the present invention, a continuous highly agitated turbulent foam is formed in the column, the bubbles of which are formed having a relatively uniform bubble size. These bubbles are constantly slightly reduced in size by dissolution of part of their contents into the liquid of the foam.

Larger bubbies, formed by coalescence, rise to the top of the foam and these are constantly reformed with entrainment and dispersion of inlet gas to replenish the supply of substantially uniform size bubbles. These processes prevent the accumulation of gas above the foam. Within the foam, bubbles move in all directions. It is surprisingly found that there is no great tendency for smaller bubbles to move downwards in the column and pass out in the exit stream; rather, there is a tendency to coalescence of such smaller bubbles to larger bubbles which rise up in the column. The phenomena of dispersion, dissolution and coalescence occur simultaneously and continuously while the method of the invention is being carried out.

The critical feature of the present invention is as noted above the introduction of one or more high speed streams of liquid into the top of the column. This naturally requires appropriate geometery both of the top of the column and of the liquid inlet but there is no particular difficulty in arranging the geometry appropriately. Generally speaking a narrow inlet pipe or a simple constriction at the end of an inlet pipe suffices to inject the incoming liquid at an appropriate high velocity, though for specific applications and indeed for optimum efficiency it may well be desirable to provide an appropriately hydrodynamically designed nozzle. In particular, the use of an appropriately designed nozzle may reduce to a minimum the energy requirement to provide the necessary reentrainment. In air or oxygen dissolving applications into water it is usually found that a liquid inlet velocity stream of speed greater than 200 cm/second is sufficient to function satisfactorily.

The geometry of the top of the column is found to be capable of surprisingly wide variation without adversely effecting the reentrainment possibilities. In the case of the simplest column, a cylindrical column, the upper end may be a flat plate perpendicular to the axis of the cylinder. If the cylinder is of large diameter, a plurality of inlets or inlet nozzles may be provided in the plate giving a plurality of high speed liquid injection streams. In place of a flat plate, the upper end of the column may be dome shaped or frustoconical. The liquid stream may be introduced vertically and tangentially. The column may be of cross section other than circular, e.g. rectangular, square or annular, and in such a case a plurality of inlets may be distributed across or around the top of the column.

It is often convenient to introduce the gas into the liquid upstream of the introduction of the liquid into the column but there is no particular criticality about the way in which the gas is introduced into the liquid feed stream.

For example a simple T-joict may be used, with the branch connected to a supply of gas under pressure. Alternatively the gas may be injected into the stream by way of an injection pipe which may point upstream or downstream relative to the liquid flow. Alternatively a known injection device e.g. a venturi injector or the like may also be used. It is preferable to have the injection of the gas, if into the incoming liquid intake stream, take place only just before the stream enters the column, since it is found that the gas liquid ratio is then more constant and more able to entrain and redisperse any bubbles or coalesced gas at the top of the column. Alternatively the gas may be fed into the suction side of an appropriate pump. With this method the pressure required to introduce the gas may be substantially reduced.

During operation the upper part of the column is filled with a continuous foam which is in a state of high agitation and turbulence.

It is believed that there is a constant bubble coalescence and breakdown accompanied by a constant redispersion caused by the incoming liquid streams. Large bubbles rising toward the top of the column tend to coalesce but the incoming stream or streams breaks up such large bubbles and redisperses the gas in them together with the addition of further gas introduced in the inlet stream or introduced otherwise into the column. The incoming liquid stream provides this redispersion effect and additionally provides the turbulence flow and backmixing which leads to very thorough gas/liquid contacting.

If desired, in order to increase the efficiency of the gas/liquid contacting yet further the column may be wholly or partly filled with an appropriate packing.

The term packing is to be construed widely in this connection as including any structure of the nature providing a high free voidage and a large number of interstices and extending across the column. Thus for example, the packing may simply be one or more sheets of mesh or perforate material. Alternatively the packing may be a bed of random or regularly arrayed packing elements of high free voidage e.g. Pull rings (Registered Trade Mark), Raschig rings or parallel sheets and static mixer packings. In this latter case, the bed may interact with the liquid, and optionally with the gas also, in a physical way or it may act in a physico-chemical way e.g. if the packing is a catalyst or supports a catalyst. The position of the packing within the column may vary widely and it may be located in a region of high back mixing and turbulence of the foam near the top of the column or it may be located further down the column in regions of lesser turbulence or even in regions where there is substantially no backmixing and little turbulence. It is surprisingly found that the introduction of a high free voidage packing into a column operated according to the present invention does not lead to a very substantial increase in coalescence of the dispersed gas phase in the foam or ro instability of the overall process.

Furthermore, the column may contain a mobile packing, e.g. a plurailty of bodies of a density and configuration such that they move about in the foam and liquid in the column and enhance the mass transfer or reaction taking place. Such a packing may consist of bodies which act purely mechanically, or which e.g. are coated with a catalyst or with biologically active material such as an enzyme. Small plastics beads or other low density particles, e.g. of density substantially the same as the liquid being treated in the column, may be used. Naturally, care must be taken to retain a mobile packing in the column. Low density mobile packings are of particular value in biological filtration and enzyme reactor applications of the present invention.

A very wide variety of apparatus may be employing in carrying out the method of the present invention. Generally speaking the apparatus can be simple and straightforward and consist basically of an absorber and/or reaction column with means for introducing liquid as a high velocity stream at the top and means for introducing gas into the column either via that high velocity stream of liquid or by means of a suitable inlet in the column. The column must naturally be provided with suitable pipework and a pump will normally be the most convenient way of passing the liquid into the column.

Although the method of the present invention generaily runs in exceptionally stable fashion, in any practical application in process technology it is customary to use appropriate control systems of known type. In this connection, it is particularly useful to contact in parallel with the column containing the gas and liquid being treated a separate column which acts as a so-called dead leg, i.e. a column in which no flow takes takes but in which a liquid level is established which is equivalent to the quiescent liquid level which would be established in the column itself were it not for the dynamic conditions which generate the dispersed gas phase. The level of the liquid in the parallel column may be used with an appropriate sensor to control e.g. gas and liquid feed rates to the main absorber and/or reactor ccl- umn. The actual control systems employed may vary widely from highly sophisticated feedback control systems with infinitely variable valves to simple systems which act to shut off the process if there is any radical departure from the standard conditions which are set manually and at the same time indicate e.g. by way of an audible alarm that the apparatus requires attention.

The control system as just noted is advantageous in practice from a number of points of view. In particular, the system may be arranged to keep the equivalent liquid level constant within narrow limits, and accordingly it serves to keep the volume of the gas bubbles in column in which the absorbtion and/or reactions are occurring and hence the mass tranfer inter facial area very constant also.

This enables control, e.g. of the desired final concentration of dissolved gas, to be achieved in a simple, effective, and elegant fashion, and enables adjustments to be made very fast in response to changes in conditions.

The method of the present invention is advantageously applicable in a very wide variety of practical processes. Particular advantages of using the method include the ability to dissolve gases totally into liquids with a minimum expenditure of energy and a high contact efficiency. This is of value in e.g. the carbonation of water in the manufacture of carbonated beverages. Another particular advantage is the insensitivity of the process to the presence of suspended solids in the feed gas fluid streams in many earlier proposals, the presence of such solids gave rise to substantial problems in practice. Using the method of the present invention the content of suspended solids can be relatively high and can vary with time, without any substantial effect on the gas/ liquid interation.

A further advantage of this method of the present invention is the virtual elimination of fouling in the column. Because of the very high degree of turbulence in the column, deposits of solids do not build up, either in the column itself or on any fixed or mobile packing therein, even when the feed gas stream contains appreciable quantities of suspended solids.

Yet a further advantage is in the close approach to equilibrium achieved using the method of the invention. In many gas absorption apparatus, a high excess pressure is needed in the gas in order to achieve the desired concentration to force it into the outlet liquid.

Using the method of the present invention, satisfactory absorption can be achieved with low pressure excess over the equilibrium pressure required. For example, in dissolving oxygen into water at 100C to a concentration of 50 ppm w/w, pressures of 2.4 kg/cm2 are common, but using the method of the present invention, pressures of about 1.8 kg/cm2 can be used, the equilibrium pressure being about 1.75 kg/cm.

The invention is illustrated, by way of Example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic side view of a simple treatment column, Figure 2 is a diagrammatic side view of a treatment column including a settling section, Figure 3 is a diagrammatic side view of an alternative configuration, Figure 4 is a diagrammatic side view of a further alternative configuration, Figure 5 shows in diagrammatic side view of a column having an intermediate takeoff point, for use when the gas/liquid ratio is above solubility ratio, Figure 6 is a view similar to Figure 5 of an alternative configuration, Figure 7 shows a further apparatus in diagrammatic side view, Figure 8 is a diagrammatic side view of a packed column for use in gas dissolution, Figure 9 is a view of the base of the column of Figure 8 slightly modified, Figure 10 is a schematic side view of a simple catalytic bed reaction column, and Figure 11 is a diagrammatic side view of a two-stage catalytic reactor including a first column located inside a second column.

In each of the drawings, bubbles are drawn schematically and are drawn distributed. In practice, during operation, a continuous foam phase is formed, especially near the top of the column, with bubbles effectively adjacent and with an appearance of a white solid dense evenly structured foam in a state of violent agitation.

Referring to the drawings: Figures 1, 2. 3, 4 and 7 each show absoIp- tion and/or reaction columns which can be operated e.g. with water as the liquid with a liquid inlet velocity exceeding 200 cm/second and with an overall liquid velocity down the column of 3 to 15 cm/second depending on bubble size. In these circumstances, the column remains entirely full of liquid and gas bubbles and no separation takes place giving any free space at the top of the column. Such arrangements have very high effectiveness for dissolving gases in liquids, and because any bubbles which do rise to the top are re-entrained by the incoming high speed stream of liquid and gas, there is total dissolution of all gas injected into the column.

If the columns are operated with water inlet velocities much greater than 200 cm/second and the downflow velocity in the body of the column is greater than 40 cm/second, the system still exhibits remarkable stability with a very turbulent flow pattern visible with the upper regions of the column. Bubbles of gas are seen to be flowing downwards and upwards in a series of circulating, changing, backmixing flow patterns, and this despite the fact that the overall flow down the column may be as high as 40 cm/second or greater while the normal rate of rise of a bubble in the liquid in the column may be only about 30 cm/second. It is found very surprisingly that bubbles are not in operation swept away down the outlet at the base of the column because of the circulating and backmixing flow pattern which is established.

Figures 1 and 2 show a 15 cm diameter column approximately 90 cm high. This can be operated according to the invention at a flowrate of approximately 9000 litres of water per hour and operating at a gauge pressure of 0.3 to 2.0 kg/cm2 to dissolve oxygen in water.

With an oxygen concentration of the water feed inlet of 11 ppm w/w, a nitrogen concentration of 19 ppm w/w, a gauge pressure of 0.9 to 1.5 kgjcmS and at a temperature of 10 C an outlet concentration of 50-75 ppm w/w is achieved with substantially 100 /o usage of the oxygen fed in.

The inlet water velocity in the 2.54 cm pipe 1 line was approximately 490 cms per second and oxygen gas was introduced by a 5 mm bore copper tube 3 bent in the direction of flow.

The high velocity water stream entering tile column sets up a strong localised circulating backmixing flow pattern of circulating backmixing streams at the top of the column in which, compared with prior art processes, the dispersion of oxygen or gas is of relatively low density and relatively high bubble packing density. The accumulation of oxygen or gas at the top of the column is prevented since larger bubbles formed by coalescence rise to the top of the column and are then entrained and their contents redispersed by the high velocity inlet stream.

In Figure 2, the base of the column 2 has a 22.5 cm diameter section 5. This minimises any entrainment of minute bubbles in the exit water stream. Deentrainment of bubbles from the exit stream may be further enhanced by withdrawing it at one or more points tangentially from the side of the column near its base. The resultant circulation of the liquid in the base of the column acts as a centrifuge or cyclone separator to force the bubbles into the centre of the column and avoid their entrainment and escape in the exit stream.

The base of the column also includes an additional gas distributor inlet 9.

Figures 3 and 4 illustrate units as Figures 1 and 2 but where the water enters a cone 6 before entering the column. Cones with walls inclined at 3 , 5 , 15" and 25 may all be used with little difference in the column performance between these types. It is found that for larger columns i.e. greater than 7.5 cm diameter an inlet water velocity of greater than 200 cms/sec is required to operate according to the invention. For smaller columns the inlet water velocity can be less than 200 cms/sec if the angle of inclination is small i.e. 3"--5".

In certain processes, it is desirable to have an excess of gas mixture or gas beyond that required for partial and/or complete solution.

Such processes include oxidation, chemical reactions and stripping using air, ionised air, oxygen, ozone in air or ozone in oxygen. In such cases, some of the bubble dispersion or foam can be passed to a separator 7 and the liquid recycled if necessary. This is illustrated in Figures 5 and 6. The removal of part of the foam can take place from any convenient part of the column, but it is to be observed that removal is of foam, rather than a venting of gas only from the column, so it is necessary in such cases to use a separator or have some arrangement which gives separation of the withdrawn foam. After separation the gas may be recycled e.g. by introduction into any appropriate part of the column optionally via a pump.

One or more bubble columns can be mounted on a common separator vessel 8, such as shown in Figure 7.

The pressure prevailing in the column 2 may vary from near atmospheric to many atmospheres. In many applications the pres sure is 0.35 Kg/cm2 gauge to 3.5 Kg/cm2 gauge.

All the above specific figures for liquid flows relate to the use of water. Especially in chemical reaction applications of the present invention, other liquids will be used with properties different from that of water. Simple experiments can be carried out to determine what flow is necessary to operate the column according to the invention.

Referring now to Figure 8, this shows a column 11 having a liquid inlet 12 at its head.

In inlet 12 is a tube or nozzle 13 into which gas may be injected from a source 14. The centre of the column is filled with a packing 16 of 25 mm Pall rings (Registered Trade Mark) to within 15 cm or more of the top of the column. Using an inlet velocity of water of > 200 cm/sec it is possible to dissolve gases in liquid streams obtaining high approach to equilibrium conditions with approximately 100% efficiency on gas utilisation. It is found that the presence of high voidage packing, e.g. the Pall rings or other packing elements in the foam increases the rate of solution of a gas in the liquid, compared to the same column operating under the same flow conditions with no packing present. Increases in the rate of 15--30C can be obtained when dissolving oxygen into water.

The packing is supported on a supporting grid 18. This should have a high free voidage if coalescence of any bubbles 17 passing through the grid is to be avoided. If the supporting grid does, in certain operating conditions, tend to act in this way, then the base of the column needs to be modified as shown in Figure 9 in order to enable gas which collects to be vented via a vent pipe 19. The presence of the packing in the foam increases the turbulence in the column and gives e.g. increased solubility of a gas in the liquid for a given foam volume. However, the packing should not extend throughout the foam region: the top of the column should be free from packing in order that the incoming high velocity stream of liquid can set up cyclic flow patterns serving to redisperse any gas bubbles which would otherwise coalesce and collect as a free space at the top of the column.

Referring now to Figure 10, a column 20 has a liquid inlet 21 at its head and a gas inlet 22 at one side. Inlet 22 terminates in a spray head 23. Below head 23 is located a bed of catalyst material 24 supported on a grid 25.

Located below grid 25 in the side of the column is a gas vent 26 and the base of the column is provided with a liquid outlet 27.

When such a column is operating, the gas introduced via head 23 after rising up the column is rapidly dispersed by the incoming high speed stream of liquid from inlet 21 and the column above the catalyst bed is full of turbulent foaming mixture of liquid gas. After passing through the catalytic bed 24 and support 25 (with a low free voidage) however, if excess gas mixture is used beyond that for partial or complete solution a coalesced gas phase forms which is vented via vent 26 and the liquid phase is removed via outlet 27.

Figure 11 shows a system in which a first column 30 is located partially inside or inside a second column 31. Liquid may be introduced into column 30 by means of an inlet pipe 32 and gas may be introduced therewith by means of a nozzle 33 fed from a source 34. At the base of column 30 is a first catalyst bed 35.

After leaving the base of column 30, the gas and liquid pass up the annulus between columns 30 and 31 through an annular second catalyst bed 36. Gas which accumulates because of the use of excess gas over that required for partial or complete solution is removed from the top of column 31 by means of a vent 37, and liquid by means of a pipe 38 located just above catalyst bed 36. If desired, further gas and/or liquid may be fed to the base of column 31 by means of a pipe 39.

If desired, the base of column 30 may be perforated above feed 35 and some liquid and gas thereby allowed to flow by-passing that bed.

WHAT WE CLAIM IS: 1. A method of contacting gas and liquid which comprises introducing the gas and liquid into a column, at least the liquid being intro duced into the top of the column in the form of at least one stream, the velocity of which stream (s) is sufficient to generate and maintain at least in an upper region of the column, a substantially continuous foam formed of closepacked bubbles of the gas in the liquid extending across the entire cross-section of the column, driven to violent agitation and backmixing by the incoming stream (s) of liquid, and of relatively uniform bubble size, the velocity of the inlet stream (s) of liquid and the rate of introduction of the gas being sufficient to prevent gas accumulating at the top of the column, and removing at least treated liquid from a lower region of the column.

2. A method according to claim 1 wherein the gas is introduced into the column together with the incoming stream (s) of liquid.

3. A method according to claim 1 wherein the gas is introduced into the column otherwise than together with the incoming stream (s) of liquid.

4. A method according to any one of claims 1 to 3, wherein the column contains a packing.

5. A method according to claim 4 wherein the packing is a mobile packing.

6. A method according to claim 4 or 5 wherein the packing supports a catalyst, biomass or enzyme.

7. A method according to any one of claims 1 to 6 wherein the liquid is water and the gas is oxygen o

Claims

**WARNING** start of CLMS field may overlap end of DESC **. After leaving the base of column 30, the gas and liquid pass up the annulus between columns 30 and 31 through an annular second catalyst bed 36. Gas which accumulates because of the use of excess gas over that required for partial or complete solution is removed from the top of column 31 by means of a vent 37, and liquid by means of a pipe 38 located just above catalyst bed 36. If desired, further gas and/or liquid may be fed to the base of column 31 by means of a pipe 39. If desired, the base of column 30 may be perforated above feed 35 and some liquid and gas thereby allowed to flow by-passing that bed. WHAT WE CLAIM IS:

1. A method of contacting gas and liquid which comprises introducing the gas and liquid into a column, at least the liquid being intro duced into the top of the column in the form of at least one stream, the velocity of which stream (s) is sufficient to generate and maintain at least in an upper region of the column, a substantially continuous foam formed of closepacked bubbles of the gas in the liquid extending across the entire cross-section of the column, driven to violent agitation and backmixing by the incoming stream (s) of liquid, and of relatively uniform bubble size, the velocity of the inlet stream (s) of liquid and the rate of introduction of the gas being sufficient to prevent gas accumulating at the top of the column, and removing at least treated liquid from a lower region of the column.

5. A method according to claim 4 wherein the packing is a mobile packing.

7. A method according to any one of claims 1 to 6 wherein the liquid is water and the gas is oxygen or air.

8. A method according to claim 7 wherein the incoming stream (s) of water has a velocity of at least 200 cm/sec.

9. A method according to any one of claims 1 to 8 wherein the gas and liquid are introduced at such rates that substantially 100% of the gas is taken into the liquid phase.

10. A method according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.