WO2010025909A1

WO2010025909A1 - Method and apparatus for gas-liquid mass transfer

Info

Publication number: WO2010025909A1
Application number: PCT/EP2009/006363
Authority: WO
Inventors: Maria Fernanda GARCÍA; Maria de Los Angeles GARCÍA
Original assignee: La Perla S.R.L.
Priority date: 2008-09-03
Filing date: 2009-09-02
Publication date: 2010-03-11
Also published as: ITPI20080084A1; IT1394586B1

Abstract

A method for gas-liquid mass transfer, comprising the steps of: prearranging a plurality of chambers; defining a path comprising preferably portions preferably vertical within the chambers between a first chamber and a last chamber; creating a gas flow along this path; creating a dispersed liquid stream through each chamber in a prefixed, preferably downwards direction; possibly recirculating the liquid among the chambers. The chambers are organised in a first group, where the liquid and the gas flow co-currently, and in a second group where the two streams flow co-currently. The chambers of the two groups are preferably alternately arranged. An apparatus for carrying out the method, comprising preferably vertical chambers (57, 59, 61, 63) which are run-through by gas (4) and by liquid (3) as above described, and possibly circulating means comprising recycling pumps (51, 52, 53, 54), and distributing piping (86, 87, 88, 89) connected thereto. In an preferred exemplary embodiment, the chambers are defined by external wall and inner parting walls of a container which nay have a square or rectangular cross section. The method and the apparatus are useful for treating very dispersed liquid streams (<500 micron) preventing the liquid from being significantly entrained in the gas stream, and for cleaning gas streams which contain dust particularly fine particulate (<10÷15 micron).

Description

TITLE METHOD AND APPARATUS FOR GAS-LIQUID MASS TRANSFER

DESCRIPTION Field of the invention The present invention relates to a method for performing mass transfer operations between a gaseous substance and a liquid. In particular, the method is well suited for absorbing a gaseous component into a liquid and for cleaning a gas from particulate solid. Furthermore, the invention relates to an apparatus for carrying out the method. When a gas comes into contact with a liquid, mass transfer may occur, due to the solubility of one or more component of the gas in the liquid; a chemical reaction may also take place between a soluble component and a substance present in the liquid, which enhances the absorbing rate. An absorbing operation may be carried out to purify a process offgas or an exhaust gas from a combustion device, or to obtain a solution which contains the gaseous component itself, or a product of a reaction of it.

Background of the invention

When a mass transfer operation has to be performed from one phase to another, an interface must be created between the phases. Preferably, the phase where the mass transfer resistance is higher should be as disperse as possible in the other phase. Examples of devices where the liquid phase is dispersed are packed columns and spray towers. As well known, packed columns are expensive and somewhat difficult to operate, while spray towers are among the simplest and cheapest gas-liquid contacting equipment: they comprise a container inside which liquid distributors, or sprayers, are arranged to provide a rain that falls down through a generally upwards-directed gas stream. In a spray tower, however, the gas speed should not be too high and/or the liquid droplets should not be too small, to avoid the entrainment of a relevant part of the liquid by the gas stream, in which case a further gas- liquid operation could become necessary downstream the spray tower to separate the entrained liquid from the gas. As a rule of thumb, to reduce liquid entrainment below acceptable limits, a gas speed lower than 1 ,2 m/s should be adopted; a larger cross sectional area would then be needed to treat a predetermined gas flow rate, thus increasing, in particular, capital and maintenance costs. Another drawback of the spray-towers is that they cannot effectively treat gas streams that contain small solid particles; it is practically impossible to remove particles whose diameter is less than 15 micron, i.e. particulate matter.

Summary of the invention

It is therefore an object of the present invention to provide a method and an apparatus for mass transfer operations between a gas and a liquid, in particular an apparatus for absorbing a component from a gas into a liquid, in which the liquid is a dispersed phase formed by small droplets, in particular, droplets smaller than 500-1000 micron, and in which no relevant entraining of the liquid in the gas stream takes place. It is, furthermore, an object of the present invention to provide such a method and such an apparatus that allow absorbing a gas into a liquid when the liquid phase resistance to mass transfer is much higher than the gas phase resistance, without using a packet bed to bring the two phases into contact.

It is a further object of the present invention to provide a method and an apparatus for removing small solid particles from a gas, in particular solid particles that are smaller than 10÷15 micron, by absorbing them into a liquid.

It is still an object of the present invention to provide a gas-liquid mass transfer apparatus, which is easy to install and to operate, which overcomes the above-mentioned spray-towers drawbacks. These and other objects are achieved by a an apparatus for mass transfer operation between a gas and a liquid, the apparatus comprising at least a first chamber and a second chamber, each of the chambers having:

- a gas inlet port and a gas outlet port for the gas;

- an inlet section and an outlet section for the liquid; - a distribution means for distributing the liquid in the inlet section, such that the liquid interacts with the distribution means forming a dispersed liquid stream which moves in each of the chambers according to a predetermined direction. The main feature of the apparatus is that a passage is provided between the gas outlet port of the first chamber and the gas inlet port of the second chamber, such that the gas flows between the gas inlet port of the first chamber and the gas outlet port of the second chamber, and flows in the first chamber according to a direction equal to the direction of the dispersed liquid stream, and flows in the second chamber according to a direction opposed to the direction of the dispersed liquid stream, or vice versa.

The apparatus may comprise an even or an odd number of chambers, preferably the apparatus comprises one, two, or three modules, each of them consisting of two chambers, as above described.

In particular, the liquid falls by gravity in the chambers, which have an elongated shape and are vertically arranged, and the gas flows upwards the first chamber and flows downwards the second chamber, or vice versa.

By such apparatus, the contact surface between the gas and the liquid can be increased by adding further series-connected modules, without using packed beds and the like.

Furthermore, the gas is obliged to follow a labyrinth path, which makes it possible to set a residence time that is adapted to a specific process, and an effective contact between the gas and the liquid can therefore be obtained. The above-described apparatus can operate regardless the liquid droplets diameter. In particular, the diameter may be less than 500 micron, i.e. the commonly accepted low-limiting value of spray-towers, because the labyrinth path through the series-connected chambers prevents liquid to be entrained by the gas that leaves the apparatus. For the same reason, a gas speed higher than 1 ,2 m/s can be used, and a prefixed column cross section can therefore effectively treat a greater amount of gas, than a conventional apparatus as a spray-tower can do.

The relatively small diameter and/or height of the chambers assists the construction of the apparatus, which is far easier than in the case of spray- towers; furthermore, the small height of the chambers enhances wind and/or seismic stability of the apparatus, because a predetermined path length, that is necessary to carry out the absorption up to a desired extent, can be distributed in a plurality of parallel arranged chambers. - A -

Small solid particles, in particular particulate smaller than 15-20 micron, can be effectively wet and therefore removed from the gas stream due to the high residence time that is allowed by the labyrinth path.

Advantageously, a liquid circulating means is provided for circulating the liquid from an outlet section to the distribution means in an inlet section, the liquid circulating means preferably comprising a pump and a distributing piping connected to the pump. This way, the liquid can be brought into contact more than one time with the gas stream: for example, in an absorption process of a component from a gas stream into a liquid, the apparatus allows to exploit the liquid solvent power up to any predetermined convenience limit. In particular, if the absorption is a steady-state process, i.e. a process where the liquid is stationary fed into and withdrawn from the apparatus according to substantially equal feeding and withdrawing flow rates, the circulating means allows to set a contact time between the phases, according to what is necessary to obtain a desired process response.

In a preferred exemplary embodiment, the first chamber and the second chamber are defined by a container and by a partition element within the container, the passage being a through hole made on the partition element. This exemplary embodiment is particularly adapted to treat low gas/liquid flow rates, for which case a compact solution is thus provided; in case of small units, a square or rectangular cross section of the container is preferred, to reduce construction time and costs.

Advantageously, the or each chamber has a bottom and the gas inlet/outlet port is located in a lower portion of the chamber and is far enough from the bottom such that a prefixed head of the liquid is formed above the bottom and below the gas inlet/outlet port. The liquid head upstream makes safer and more stable the liquid circulating means operation; this is particularly important when dealing with a flammable or instable liquid and/or if the liquid circulating means comprises a centrifugal pump. Preferably, a connection is provided between the lower portions of the first chamber and of the second chamber, such that draining of liquid between the chambers is allowed, the head opposing a gastight seal to gas leakage between the chambers through the connection. This way, liquid collecting and draining operations to be performed the end of a production cycle are simplified.

In a preferred exemplary embodiment, the distribution means comprises a sieve plate that is arranged at the inlet section, wherein the sieve plate has through holes such that a liquid stream that has a prefixed flow rate and that is fed above the sieve plate, by flowing through the through holes:

- is turned into a dispersed liquid stream that falls down inside each chamber; forms a stationary head above the sieve plate, said stationary head providing a gastight seal against leakage of the gas through the through holes. In this way the gas is forced to follow the labyrinth path along the chambers, along the gas inlet ports of the chambers and along the passage between the chambers, and can neither escape from an open upper portion of the chamber, nor form a stationary gas blanket in a closed upper portion of the chamber. The above-mentioned objects, and other, are also achieved by an apparatus for mass transfer operation between a gas and a liquid, the apparatus comprising a plurality of chambers, each of the chambers having: a gas inlet port and a gas outlet port for the gas;

- an inlet section and an outlet section for the liquid; - a distribution means, for distributing the liquid in the inlet section, such that the liquid interacts with the distribution means forming a dispersed liquid stream which moves within each of the chambers according to a predetermined direction. The main feature of the apparatus is that a passage is provided between the gas outlet port of each chamber and the gas inlet port of another of the chambers, such that the gas flows within a first group of chambers according to a direction equal to the direction of the dispersed liquid stream, and flows within a second group of chambers according to a direction opposite to the direction of the dispersed liquid stream, or vice versa.

The above-mentioned objects, and other, are also achieved by a method for mass transfer between a liquid and a gas, the method comprising the steps of:

- prearranging a plurality of chambers,

- defining a path within the chambers between a first chamber and a last chamber of the plurality of chambers; causing a gas flow along the path; causing a liquid flow along the path in a dispersed form, the liquid having a direction in each of the chambers; wherein the gas flows in a first group of chambers in the same direction of the liquid flow, and flows in a second group of chambers in an opposite direction of the liquid flow.

Preferably, each of the chambers has only two ports connected to respective ports of other chambers, and the path is unique. In other words, the chambers are connected in series, or serially connected, and can be run- through by the gas according to this series from a first to a last chamber.

In particular, the gas flows in a chamber of the first group alternated to a chamber of the second group.

In particular, the liquid moves downwards by gravity and the path is formed by a succession of vertical path portions.

Advantageously, the method provides a step of circulating the liquid between outlet sections and inlet sections of respective chambers.

Preferably, a step is provided of prearranging an amount of the liquid in a container that is connected to a withdrawing means, in a lower portion of a chamber; the step of prearranging an amount of the liquid is followed by a circulating step, wherein the liquid is circulated to the distributing means at the liquid inlet section.

In a discontinuous operation, the liquid is neither withdrawn from nor made-up into the apparatus, and the circulating step goes on until a predetermined stop criterion occurs. Then the exhausted liquid is totally withdrawn and a fresh liquid amount is supplied to the apparatus, and a new cycle is carried out. The stop criterion may be the elapsing of a prefixed time interval after operation start, or the achievement of a prefixed concentration of the component in the liquid. In a continuous operation, a stream of exhausted liquid is continuously withdrawn from the apparatus; preferably, an equal stream of fresh liquid is continuously made-up thereto according to a prefixed liquid feeding/delivering flow rate, or according to a prefixed make-up-to-circulating liquid ratio. This way, a contact time between the liquid and the gas phase can be set in a more precise way, responsive to a desired treatment time.

In particular, the above-described method and apparatus can be used for absorbing an acid gas into an alkaline aqueous solutions, in which, in particular a salt is formed: the salt may be, for instance, a chloride or a sulphate.

The above-described method and apparatus may also be used for removing an offensively-smelling substance from a gas by absorbing the substance into a liquid; in particular, the substance may be one selected between ammonia, alcohols and volatile acid fats and/or a derivative thereof, and the liquid is water or a water solution. In alternative, but not exclusively, the substance may be one selected between chlorinated chemicals, amines, hydrogen sulphide. This gas deodorization process may be intended to purify a feed or an intermediate process stream which contains substances that might interfere with a subsequent process operation. Another possible use of the above-described method may be the manufacture of a solution or of a suspension of a useful substance by contacting a gas which contains the substance or a precursor thereof and a liquid that has a physical or a chemical affinity to the useful substance, wherein, in particular the precursor is carbon dioxide, the liquid comprises water and the useful substance is calcium carbonate which is formed by reaction of the carbon dioxide and the water.

The liquid may contain a chemical which is adapted to react with the component to be absorbed, which enhances the absorption rate. Brief description of the drawings The invention will be made clearer with the following description of some exemplary embodiments, exemplifying but not limitative, with reference to the attached drawings, in which like reference characters designate the same or similar parts, throughout the figures of which:

- figure 1 shows a first exemplary embodiment of the apparatus according to the invention for absorbing a component from a gas stream into a liquid;

- figure 2 shows more in detail, and without process fluids, the apparatus shown in figure 1 ; figure 3 is a cross sectional view of the apparatus shown in figure 2; - figures 4 and 5 show respectively external and internal walls of the apparatus of figure 2; figure 6 is a view of a distributing plate for liquid in the apparatus of figure 3; - figure 7 is a flow diagram of the apparatus of figure 1 ;

- figure 8 shows diagrammatically a second exemplary embodiment of the apparatus according to the invention for absorbing a component from a gas stream into a liquid; figure 9 is a flow diagram of the apparatus of figure 8; - figure 10 shows, more in detail, an equipment of the apparatus depicted in figure 9;

- figure 11 shows diagrammatically a spraying nozzle for distributing the liquid as in the apparatus of figure 9, with the relative spraying cone.

Description of preferred exemplary embodiments With reference to figures 1 to 7, an apparatus 100 for absorbing a gas component from a gas 1 into a liquid 2 is described, according to a first exemplary embodiment of the invention, in which a multiple absorber 30 comprises a container 31 (figures 2 and 4) that has the shape of a rectangular parallelepiped with a gas inlet port 6 and a gas outlet port 14 for gas 1. A rectangular or square cross section assists constructing the unit, and is particularly adapted to make small size equipment; at any rate, may also have a different shape, for instance a cylindrical shape. A cross-shaped parting element 36 (figure 5) is vertically arranged in container 31 , and consists of two couples 37 and 38 of wings or parting walls arranged to form a cross, which define, together with the external walls of container 31 , four absorption chambers 7,9,11 ,13 (figure 3) that have equal rectangular cross section. Through three parting walls of the cross-shaped parting element 36 corresponding holes 8,10,12 are made (figures 2 and 5) to allow gas flow from chamber 7 to chamber 13, through chambers 9 and then 11. The position of holes 6,8,10,12,14 is such that the gas, moving under a differential pressure between hole 6 and hole 14, is obliged to follow a labyrinth path, i.e. a zig-zag path upwards inside chambers 7 and 11 , and downwards inside chambers 9 and 13. Furthermore, container 31 has two holes 18 and 19, which are centred in the upper face 25 and in the lower face 26, to respectively allow liquid inlet and outlet. On top of cross-shaped parting element 36, a plate distributor 40 is arranged (figures 2 and 6) which is provided with holes 41 , for example with circular holes that are arranged to form parallel arrows. The diameter and the number of holes 41 is predetermined in such a way that, by feeding a liquid through hole 18 according to a predetermined recycling flow rate, a substantially steady liquid head 32 of liquid (figure 1) is formed on plate 40 at a height b with respect to plate 40. This liquid flows through holes 41 of plate 40 preferably forming a substantially uniform rain in chambers 7,9,11 ,13. Furthermore, liquid head 32 prevents gas from passing through the liquid. In other words, liquid head 32 provided a liquid distributing means as well as a gas-tight sealing means; the gas is forced to pass through holes 8 and 12 and is prevented from forming stationary gas blankets in the upper part of absorption chambers 7 and 11 , or from leaving the chambers by means of an opening possibly made in the upper part of the chambers, as provided for in an alternate, not shown true atmospheric exemplary embodiment. Instead of plate 40, the liquid distributing means may be provided by a nozzle distributor 90, as schematically depicted in figure 11.

Therefore, a counter current, dispersed flow of liquid 3 and gas 1 is formed in chambers 7 and 11 , while a dispersed, co-current liquid-gas flow takes place in chambers 9 and 13 (figure 1). In this exemplary embodiment, liquid 2 is parallel fed to chambers 7,9,11 ,13, i.e. a liquid stream is split into four streams that are fed to corresponding chambers.

The liquid of stream 3 is collected in a lower portion of each chamber, thus creating a bottom liquid head 33. Each chamber lower portion communicates through a passage port 35 that is made through the walls of cross-shaped parting element 36 (figure 5), to allow homogeneous mixing of the liquid that is collected in liquid head 33. Once liquid bottom head 33 has grown up to a predetermined height p, a a gas-tight sealing is provided between chambers 7,9,11 ,13, which prevents the gas from leaking from a given chamber into the subsequent chamber, according to gas path, the subsequent chamber being at a lower pressure than the given chamber.

The liquid collected inside the bottom part of chamber 7,9,11 ,13 can reach outlet hole 19, which is connected to the suction port of a pump 16 by a duct 20. A duct 17 is arranged at delivery side of pump 16, which allows recycling the liquid withdrawn from bottom liquid head 33 to the distributing means, in particular upon distributing plate 40 of absorption chambers 7,9,11 ,13. Two valves 21 and 22 allows switching the suction side of pump 16 from multiple absorber 30 to a liquid feeding means, not shown, which is connected to multiple absorber 30 by a duct 34. Similarly, two valves 23 and 24 allows switching the pressure side of pump 16 from multiple absorber 30 to a liquid collecting means, still not shown, for example a storage tank, connected with the apparatus through a duct 29.

A flow-sheet of an apparatus 100 is shown in figure 7, wherein valves 21 ,22 and 23,24 are omitted and instruments 91 and 92 are shown for directly or indirectly measuring the concentration, respectively in the gas and in the liquid, of a component which is transferred from gas 1 to liquid 2 by means of apparatus 100.

Apparatus 100 can be used in batch mode, as below described with reference to figures 1 and 7: a) an amount of liquid 2 is fed to apparatus 100 through duct 34, and is collected in a lower portion of chambers 7,9,11 ,13. Bottom liquid head 33 of liquid is therefore provided (figure 1 ; obviously, liquid 2 can be fed through a nozzle, not shown, made on a side wall of container 31); thereafter, b) liquid 2 is recycled or circulated through pump 16 between the outlet section at the bottom of the chambers 7,9,11 ,13, and the distribution means of the same chambers 7,9,11 ,13, according to a prefixed flow-rate. Each chamber treats a dispersed liquid stream 3. At the same time, gas 1 is fed through a duct 5 and hole 6 to multiple absorber 30, which is run through by the gas according to the zig-zag path defined by chambers 7,9,11 ,13 and by passage ports 6,8,10,12,14, a two-phase flow is therefore created in chambers 7,9,11 ,13; more in detail, a counter-current flow is established in chambers 7 and 11 , whereas and a co-current flow takes place in chambers 9 and 13. The recycling flow rate of the liquid is decided according to the process, typically according to the liquid solubility of the component to be absorbed from gas 1 to liquid 3; as a rule of thumb, however, a reference minimum value, which applies for a large number of absorption processes, corresponds to ten recycles/hour of the whole amount of liquid contained in the apparatus. c) the recycling step is kept on until a stop condition occurs, which may be: achieving a desired concentration of the component adsorbed in the liquid, when the purpose of the process is to obtain a liquid solution of this component, or a liquid solution of a product formed by interaction between the component and the liquid; reducing the gas concentration of the component below an admissible threshold, when the purpose is to obtain a gas substantially free or containing a small concentration of the component, which is the case of cleaning a gas stream delivered to atmosphere or sent to further process operation wherein a high concentration of the component cannot be tolerated; carrying out a prefixed number of liquid recycles, or a prefixed treatment time under predetermined flow conditions, as may be suggested by a deep knowledge of the process.

Gas and/or liquid phase concentrations are monitored through respective instruments 91 and 92, according to a sampling frequency that depends on the instrument and the process. In particular, the liquid phase concentration can be directly measured, or deducted from an auxiliary variable which is easier to evaluate than concentration, as in the case of torbidity or density, provided these physical quantities are related with concentration in an enough definite way, or the desired process result is formulated with reference to these physical quantities. d) gas feeding to multiple absorber 30, as well as liquid recycling is suspended in particular, by opening valve 24 and closing valve 23, so that the liquid is withdrawn from multiple absorber 30 and sent to a liquid collecting, not shown, means through pump 16 and duct 29.

In some cases, for example when a waste or intermediate gas stream of a steady state process has to be treated, or when an exhaust gas produced by continuous combustion plant has to be purified, the absorption must be continuous as well. Apparatus 100 can still be used in batch mode, as above described, but an apparatus comprising at least a first and a second multiple absorbers 30 is then needed, as well as a means for switching the gas stream from the first to the second apparatus and vice versa, to be used at the end of each treatment cycle. More advantageously, in this case, apparatus 100 can be used according to a continuous mode, wherein liquid streams are continuously supplied and extracted from multiple absorber 30, respectively, via ducts 34 and 29. In this case, if apparatus 100 operates regularly, the component concentration is substantially unchanged along a whole treatment cycle both in the gas and in the liquid streams. More in detail, a) an amount of fresh liquid 2 can be loaded into multiple absorber 30 at the beginning of the treatment cycle through duct 34, providing a bottom liquid head in a lower portion of chambers 7,9,11 ,13, as already described; thereafter, b) the liquid is recycled or circulated through pump 16 between the outlet section at the bottom of the chambers 7,9,11 ,13, and feeding hole 18, so that chambers 7,9,11 ,13 are run-through by a dispersed liquid stream, i.e. by a substantially uniform rain 3 of liquid; c) at the same time, gas 1 is fed to multiple absorber 30 through duct 5 and hole 6, which is run through by the gas according to the labyrinth path along chambers 7,9,11 ,13 and along passage ports 6,8,10,12,14, a two-phase flow is therefore created in chambers 7,9,11 ,13, more in detail a counter- current flow in chambers 7 and 11 , and a co-current flow in chambers 9 and 13; d) feeding of fresh liquid is carried out according to a predetermined liquid feeding flow rate through duct 34; exhausted liquid is extracted from the apparatus by means of duct 29 according to a liquid delivering flow rate substantially equal to liquid feeding flow rate. The ratio between the recycling flow rate and the feeding/delivering flow rate is predetermined according to the absorption process, and can be kept fixed or adjusted by means of an automatic control system, not shown, which adjust opening stroke of valves 21 ,22 and 23,24. In particular, an adjusting step can be provided wherein the ratio is adjusted according to liquid and/or gas phase component concentration, or according to a property which is related with a concentration, through measuring instruments 91 and 92; in a preferred exemplary embodiment, an automatic adjusting loop is provided wherein concentration data are continuously collected from instruments 91 and 92 and sent to a data processor, which is suitable for changing the opening stroke of valves 21 ,22, 23,24 by means of an actuating device, not shown, in order to adjust make-up- to-circulating liquid ratio. d) the treatment goes further along with the production cycle of the plant delivering the apparatus 100 gas stream 1 ; when the production cycle is stopped, as in the case of maintenance operation or production turnover, the liquid is removed from multiple absorber 30 as in the above-described batch mode. Apparatus 100 so far referred to can be advantageously used for treating small streams/amounts of gas or liquid; an apparatus 700, according to a second exemplary embodiment of the invention, is more suitable for treating larger streams/amounts. Such apparatus is described below, with reference to figures 8 to 11. To absorb a component from a gas 1 into a liquid 2, apparatus 700 comprises four substantially cylindrical containers, called columns 57,59,61 ,63 (figure 8) with respective gas inlet ports 56,80,82,84 and respective gas outlet ports 79,81 ,83,85 for. Therefore, gas 1 , subject to a differential pressure between port 56 and port 85, is obliged to follow a labyrinth path, i.e. a zig-zag path, upwards inside columns 57 and 61 , and downwards inside columns 59 and 63. Gas inlet and gas outlet ports are oriented as required by plant lay-out. Furthermore, columns 57,59,61 ,63 have head-centred inlet holes and bottom-centred outlet holes, for example, column 59 has an inlet hole 67 and an outlet hole 43, which are respectively made at the centre of the upper face 25 and of the upper face 26, to respectively allow liquid inlet and outlet. Hole 67 is engaged by a spraying nozzles 59, as well as respective liquid inlet holes of columns 57,61 ,63 are engaged by respective spraying nozzles 75,77,78. These are indicated as 90 in figure 11 , and provide distributing the liquid, forming a substantially uniform rain in columns 57,59,61 ,63 Furthermore, diffusion cone 91 formed by the liquid after leaving nozzle 90 is preferably selected such that gas is prevented from attaining the upper part of each column. In other words, spray nozzles 75,76,77,78 provide a liquid distributing means as well as a sealing means; the gas is therefore forced to pass through gas outlet ports 79 and 83, or to travel columns 59, 63 and is prevented from forming stationary gas blankets in the upper part of the absorption columns, or from leaving the columns by means of an opening possibly made in the upper part of the columns, as provided for in an alternate, not shown true atmospheric exemplary embodiment. Therefore, a counter current, dispersed flow of liquid 3 and gas 1 is formed in columns 57 and 61 , while a dispersed, co-current liquid-gas flow takes place in columns 59 and 63 (figure 8).

The liquid is collected in respective lower portions of columns 57,59,61 ,63, thus creating respective bottom liquid heads 46,47,48,49. Column lower portions communicates with one another through linking ducts 64, in such a way that a substantially homogeneous mixing is provided. Once liquid bottom heads 46,47,48,49 have grown up to a predetermined height p, they provide a gas-tight sealing between columns 57,59,61 ,63, thus preventing the gas from leaking from a given column into the subsequent column, according to gas path, the subsequent column being at a lower pressure than the given column.

Each of columns 57,59,61 ,63 has respective liquid outlet holes 42,43,44,45 that are connected to the suction port of respective recycling pumps 51 ,52,53,54. Ducts 86,87,88,89 are arranged at delivery side of recycling pumps 51 ,52,53,54 which allows recycling liquid withdrawn from bottom liquid heads 46,47,48,49 of columns 57,59,61 ,63, to spray nozzles 76,77,78,75 of columns 59,61 ,63,57, respectively, so that the liquid that has less interacted with the gas is recycled to the column where the component concentration treats in gas phase is higher. A feed pipe 73 and a valve 71 are provided to allow fresh liquid from a liquid feeding means, not shown, to be charged into column 61. In particular, fresh liquid is charged into column 61 , from which it can reach the bottom portions of each columns by means of linking ducts 64, which provides also a liquid homogenizing means and allows also a single collecting and drainage point for liquid that has to be withdrawn from apparatus 700, to be used at the end of each working cycle. To this purpose, a duct 68 and a valve 52 are provided, which connect the lower parts of the columns with an extracting pump 66, whose pressure side is connected via a pipe 69 to a liquid collecting means, still not shown, for example a storage tank.

A flow-sheet of apparatus 700 is shown in figure 9, in which valves 52,71 are omitted and instruments 101 and 102 are shown for directly or indirectly measuring the concentration, respectively in the gas and in the liquid, of a component which is transferred from gas 1 to liquid 2 by means of apparatus

700.

Similarly to apparatus 100, apparatus 700 can be operated according in batch mode or in continuous mode. In the first case: a) an amount of liquid 2 is loaded into column 61 , through duct 73, in order to form in each column bottom portions respective liquid heads 46,47,48,49, by means of linking ducts 64; b) liquid 2 is recycled or circulated through recycling pumps 51 ,52,53,54 between the outlet section at the bottom of each columns 57,59,61 ,63, and the spraying nozzles 75,76,77,78, according to prefixed recycle flow-rates, preferably according to substantially equal recycle flow-rates. Each column treats a dispersed liquid stream 3. At the same time gas 1 is fed through a duct 55 and gas inlet port 56 to column 57, which is run through by the gas according to the zig-zag path defined by columns 57,59,61 ,63 and by ports 79,80,81 ,82,83,84. A two-phase flow is therefore created in columns 57,59,61 ,63, more in detail, a counter-current flow is established in columns 57 and 61 , whereas and a co-current flow takes place in columns 59 and 63. The liquid recycling flow rate is decided according to the process, typically according to the solubility of the component into liquid 3. c) the recycling step is kept on until a stop condition occurs, which may be: - achieving a desired concentration of the component adsorbed in the liquid, when the purpose of the process is to obtain a liquid solution of this component, or a liquid solution of a product formed by interaction between the component and the liquid; reducing the gas concentration of the component below an admissible threshold, when the purpose is to obtain a gas substantially free or containing a small concentration of the component, which is the case of cleaning a gas stream delivered to atmosphere or sent to further process operation wherein a high concentration of the component cannot be tolerated; carryϊng out a prefixed number of liquid recycles, or a prefixed treatment time under predetermined flow conditions, as may be suggested by a deep knowledge of the process.

Gas and/or liquid phase concentration are monitored through respective instruments 101 and 102, according to a sampling frequency that depends upon the instrument and the process. d) gas feeding to column 57, as well as liquid recycling is suspended, in particular, by stopping recycling pumps 51 ,52,53,54, by starting extracting pump 66 and by opening valves 52 and 74, so that the liquid is withdrawn from apparatus 700 and sent to a liquid collecting means, not shown, by means of extracting pump 66 and duct 69.

The operation of apparatus 700 may be continuous as well. Liquid streams are then continuously supplied and extracted from apparatus 700, respectively, via ducts 73 and 69. If apparatus 700 operates regularly, the component concentration is substantially unchanged along a whole treatment cycle both in the gas and in the liquid streams. More in detail, a) an amount of fresh liquid 2 can be loaded into column 61 at the beginning of the treatment cycle through duct 34, providing a bottom liquid head in a lower portion of columns 57,59,61 ,63 as already described; thereafter, b) the liquid is recycled or circulated through recycling pumps 51 ,52,53,54 between the outlet section at the bottom of the columns 57,59,61 ,63 and nozzles distributors, respectively, 76,77,78,75, so that columns 57,59,61 ,13 are run-through by a dispersed liquid stream 3 of liquid; c) at the same time, gas 1 is fed to column 57 through duct 55 and gas inlet port 56. Apparatus 700 is run through by the gas according to the labyrinth path along columns 57,59,61 ,63 and ports 79,80,81 ,82,83,84, a two- phase flow is therefore created in columns 57,59,61 ,63, more in detail a counter-current flow in columns 57 and 61 , and a co-current flow in columns 59 and 63; d) feeding of fresh liquid is carried out according to a predetermined liquid feeding flow rate through duct 73; exhausted liquid is extracted from the apparatus by means of duct 69 according to a delivering liquid flow rate substantially equal to feeding flow rate. The ratio between the recycling flow rate and the feeding/delivering flow rate is predetermined according to the absorption process, and can be kept fixed or adjusted by means of an automatic control system, not shown, which adjust opening stroke of valves 71 and 74. In particular, an adjusting step can be provided wherein the ratio is adjusted according to liquid and/or gas phase component concentration, or according to a property which is related with a concentration, through measuring instruments 101 and 102; in a preferred exemplary embodiment, an automatic adjusting loop is provided wherein concentration data are continuously collected from instruments 101 and 102 and sent to a data processor, which is suitable for changing the opening stroke of valves 71 and 74 and possibly of other valves, not shown, arranged along the pressure side of recycling pumps 51 ,52,53,54, by means of an actuating chain, still not shown, in order to adjust make-up-to-circulating liquid ratio. e) the treatment goes further along with the production cycle of the plant delivering gas stream 1 to apparatus 700; when the production cycle is stopped, as in the case of maintenance operation or production turnover, the liquid is removed from apparatus 700 as in the above-described batch mode.

By using the apparatus according to the invention, operations can be carried out that would otherwise require packed columns, with the above cited drawbacks. Such operations are in particular: absorbing an acid gas into an alkaline aqueous solutions, which leads to respective salts formation, in particular, to chlorides and sulphates production, preferably in the form of a solid-liquid suspension; - gas deodohzation by using a suitable solvent: for example, water can be used for ammonia, alcohols and volatile acid fats; substantially water-insoluble chemicals, as chlorinated chemicals, amines, hydrogen sulphide, can be treated provided a chemical reactant is previously arranged in the liquid phase; making useful solutions or suspension: for example, an aqueous calcium carbonate suspension can be produced by absorbing a carbon dioxide gas stream into an aqueous calcium hydroxide suspension, where the following reaction takes place:

Ca(OH)₇ + CO, → CaCO₁ + H,0 Two examples are referred to below. The feeding suspension contains 20% calcium hydroxide; taking into account the water which is produced along with the reaction, a 24% calcium carbonate suspension can be obtained.

Example n. 1 To make 100 Kg/hour 24% CaCO₃ aqueous suspension, 44 Kg/ hour CO₂ are adsorbed into 370 Kg/hour Ca(OH)₂ aqueous suspension (which correspond to 322 litres/hour) in apparatus 100. Membrane volumetric pump 16 allows a recycling flow rate of 5 m³/hour; this corresponds to recycling a charge of 322 litres 15 times in an hour. Chambers 7,9,11 ,13 have square 300 mm x 300 mm cross sections and 4000 mm overall long height h, of which a 2200 mm long central portion h₂ is available for bringing the gas stream into contact with the liquid stream. Central portion h₂ is limited by respectively 800 and 1000 mm long terminal portion hi and h₃, due to distributing plate 40 and holes 8,10,14. Height b of liquid head 32 that is steadily accumulated on plate 40, at given gas and liquid flow-rates, is 450 mm, while height p of liquid bottom head 33 that is collected in the lower portion of each chamber 7,9,11 ,13 changes from 900 mm to 450 mm during liquid circulation of the liquid, due to liquid head 32 formation.

Example n. 2 To make 1000 Kg/hour 24% CaCO₃ aqueous suspension, 440 Kg/hour of CO₂ are adsorbed into 3700 Kg/hour of Ca(OH)₂ aqueous suspension (which correspond to 3220 litres/hour) in apparatus 700. Pumps 51,52,53,54 allow each a partial recycling flow rate of 12,5 m³/ hour, thus creating an overall recycling flow rate of 50 m³/hour, which corresponds to recycling a charge of 3220 litres 15 times in an hour. Columns 57,59,61 ,63 have a to 850 mm diameter D circular cross section equal, and a 8500 mm overall height H, of which 6300 mm long portion H₂ is available for bringing the gas stream into contact with the liquid stream. Central portion H₂ is limited by respectively 1600 and 600 mm long terminal portion H₁ and H₃, due to spraying nozzles and gas inlet/outlet ports.

Height P of the bottom liquid head in the lower portion of each column is about 1400 mm. Finally, the above-described apparatuses can be used also for executing absorption operations that are associated with significant thermal effects. One or more heat exchange units may then be necessary, in the case of heat exchangers , these are preferably arranged along delivery ducts 17 (apparatus 100) or 86,87,88,89 (apparatus 700) of pumps 16 or recycling pumps 51 ,52,53,54 (figures 1 and 9).

The foregoing description of specific embodiments will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such embodiments without further research and without parting from the invention, and it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiments. The means and the materials to realise the different functions described herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

Claims

1. An apparatus for mass transfer operation between a gas and a liquid, said apparatus comprising at least a first chamber and a second chamber, each of said chambers having: - a gas inlet port and a gas outlet port for said gas;

- an inlet section and an outlet section for said liquid;

- a distribution means for distributing said liquid in said inlet section, such that said liquid interacts with said distribution means forming a dispersed liquid stream which moves in each of said chambers according to a predetermined direction, characterized in that a passage is provided between said gas outlet port of said first chamber and said gas inlet port of said second chamber, such that said gas flows between said gas inlet port of said first chamber and said gas outlet port of said second chamber, and flows in said first chamber according to a direction equal to said direction of said dispersed liquid stream, and flows in said second chamber according to a direction opposed to said direction of said dispersed liquid stream, or vice versa.

2. An apparatus according to claim 1 , wherein

- said liquid falls by gravity in said chambers; - said chambers have an elongated shape and are arranged vertically, and

- said gas flows upwards said first chamber and flows downward said second chamber, or vice versa.

3. An apparatus according to claim 1 , wherein a liquid circulating means is provided for circulating said liquid from an outlet section to said distribution means in an inlet section, said liquid circulating means preferably comprising a pump and a distributing piping connected thereto.

4. An apparatus according to claim 1 , wherein said first chamber and said second chamber are defined by a container and by a partition element within said container, said passage being a through hole made on said partition element.

5. An apparatus according to claim 2, wherein said or each chamber has a bottom and said gas inlet/outlet port is located in a lower portion of said chamber and is made far enough from said bottom such that a prefixed head of said liquid is formed above said bottom and below said gas inlet/outlet port.

6. An apparatus according to claim 5, wherein a connection is provided between said lower portions of said first chamber and of said second chamber, such that draining of liquid between said chambers is allowed, said head opposing a gastight seal to gas leakage between said chambers through said connection.

7. An apparatus according to claim 2, wherein said distribution means comprises a sieve plate that is arranged at said inlet section of each chamber, wherein said sieve plate has through holes such that a liquid stream that has a prefixed flow rate and that is fed above said sieve plate, by flowing through said through holes:

- is turned into a dispersed liquid stream falling down inside each chamber; forms a stationary head above said sieve plate, said stationary head providing a gastight seal against leakage of said gas through said through holes.

8. An apparatus for mass transfer operation between a gas and a liquid, said apparatus comprising a plurality of chambers, each of said chambers having:

- a gas inlet port and a gas outlet port for said gas; - an inlet section and an outlet section for said liquid;

- a distribution means for distributing said liquid in said inlet section, such that said liquid interacts with said distribution means forming a dispersed liquid stream which moves within each of said chambers according to a predetermined direction, characterized in that a passage is provided between said gas outlet port of each chamber and said gas inlet port of another of said chambers, such that said gas flows within a first group of chambers according to a direction equal to said direction of said dispersed liquid stream, and flows within a second group of chambers according to a direction opposite to said direction of said dispersed liquid stream, or vice versa.

9. A method for mass transfer between a liquid and a gas, said method comprising the steps of: - prearranging a plurality of chambers,

- defining a path within said chambers between a first chamber and a last chamber of said plurality of chambers;

- causing a gas flow along said path;

- causing a liquid flow along said path in a dispersed form, said liquid having a direction in each of said chambers, wherein said gas flows in a first group of chambers in the same direction of said liquid flow, and flows in a second group of chambers in an opposite direction of said liquid flow.

10. A method according to claim 9, wherein each of said chambers has only two ports connected to respective ports of other chambers, and said path is unique.

11. A method according to claim 10, wherein said gas flows in a chamber of said first group alternated to a chamber of said second group.

12. A method according to claim 11 , wherein said liquid moves downwards by gravity and said path is formed by a succession of vertical path portions.

13. A method according to claim 12, comprising a step of circulating said liquid between outlet sections and inlet sections of respective chambers.

14. Use of the apparatus according to the claims from 1 to 8, for carrying out a process selected among the group comprised of: absorbing an acid gas into an alkaline aqueous solutions, in which, in particular a salt is formed, said salt being in particular a chloride or a sulphate; deodorization of a gas, i.e. removal of a smelling substance from a gas by absorbing it into a liquid, wherein, in particular, said smelling substance is selected among the group comprised of ammonia, alcohols and volatile acid fats and/or a derivative thereof, and said liquid is water or a water solution; wherein, in particular said smelling substance is selected among the group comprised of chlorinated chemicals, amines, hydrogen sulphide and/or respective derivative, and said liquid is an organic solvent or an organic solvent based solution; - making a solution or a suspension of a useful substance by contacting a gas which contains said useful substance or a precursor thereof and a liquid which has a physical or a chemical affinity to said useful substance, wherein, in particular said precursor is carbon dioxide, said liquid comprises water and said useful substance is calcium carbonate which is formed by reaction of said carbon dioxide and said water.