MIST REACTOR OF THE VERTICAL FLOW TYPE, AND USE THEREOF

Mist reactor of the vertical flow type, and use thereof

The present invention relates to a mist reactor of the vertical-flow type, comprising: - a reactor chamber; atomization means which are arranged in the top of the reactor chamber, for atomizing a liquid in the reactor chamber to form a mist; an inlet, which opens out at the bottom of the reactor chamber, for feeding a gas stream, which is to be brought into contact with the mist and is to be passed through the reactor chamber, to the reactor chamber; and an outlet, at the top of the reactor chamber, for the gas stream which has been passed through the reactor chamber.

Mist reactors of this type are known and are used to bring a gas stream into contact with an atomized flow of liquid in order to achieve mass transfer from the gas phase to the liquid phase. Known reactors of this type are used, for example, to clean gases by atomizing a suspension of micro-organisms therein. The drops absorb or adsorb impurities which are present in the gas stream and which are then broken down by the micro-organisms. The liquid or suspension is atomized in the top of the reactor and drops downwards under the force of gravity. The gas stream flows through the reactor in the upward direction, from the bottom towards the top. The upward gas stream decelerates the falling atomized drops. To keep the mist in the reactor as long as possible, it is advantageous if the velocity of the gas stream and the droplet size are adapted to one another in order to be able to effect a type of suspension of the drops. In a suspension of this type, the speed at which the drop falls is equal or approximately equal to the upward velocity of the gas.

EP-A 0 853 975 discloses a reactor for the production of "polyadducts of alkylene oxides with a combined liquid-in-gas and gas-in-liquid dispersion reactor". This is not a reactor of the vertical-flow type. It is true that EP-A 0 853 975 discloses a gas inlet which opens out in the bottom of the reactor, but it does not disclose a gas outlet and therefore also fails to disclose a gas outlet in the top of the reactor which is required for gas to flow through vertically. The gas supplied in EP-A 0 853 975 is a reactant which disappears entirely in a reaction with the aqueous phase.

However, a problem which occurs in practice is that the atomizers which are used, such as spray nozzles, produce drops of uneven size. Although atomizers which produce drops of similar size are known, in practice atomizers of this type are unable to produce drops of identical size (even though they are intended to). In practice, atomizers of this type also produce a certain particle size distribution with regard to the atomized drops. The smaller the particle size distribution which can be achieved with the known atomizers, the more expensive these atomizers generally become. In practice, the drops which are too small will be entrained. by the gas stream, and the drops which are too large will drop downward in the opposite direction to the gas stream. To be able nevertheless to ensure sufficient contact between the mist and the gas stream, it is possible for the height of the reactor chamber in the known mist reactors to be selected to be sufficiently great for the distance over which the drops fall to increase. Atomized drops which do not remain in the reactor chamber or do not remain in the reactor chamber for a sufficiently long time, i.e. do not become suspended, contribute to lowering the efficiency of the known mixed reactors. A further problem is that the size and/or weight of the mixed drops during residence in the reactor chamber may increase and/or decrease as a result of processes which take place in the reactor chamber, such as coalescence, evaporation and splitting (breaking up) of the drops. This will lead to suspended atomized drops, in the event of their size and/or weight increasing, nevertheless dropping downward or to suspended atomized drops being entrained by the gas stream as a result of their size and/or weight decreasing.

It is an object of the present invention to provide an improved mist reactor which overcomes a large number, if not all, of the abovementioned drawbacks.

According to the invention, this object is achieved by the fact that, from the bottom upwards, the horizontal cross-sectional area of the reactor chamber increases. If the cross-section area of the reactor chamber increases from the bottom upwards, i.e. in the direction of flow of the gas stream, the velocity of the gas stream will decrease accordingly. This means that drops with a smaller diameter and or a lower weight will be suspended at a higher level in the reactor chamber, in relative terms, while drops with a larger diameter and/or a greater weight will be suspended at a lower level in the reactor chamber, in relative terms.

According to the invention, the term mist is understood as meaning in particular a dispersion of a liquid or suspension in gas, the diameter of the liquid/suspension particles being less than 1500 μm, more particularly less than 1000 μm, preferably in the range from 1 μm to 600 μm, and more preferably in the range from 100 μm to 500 μm.

According to an advantageous embodiment of the mist reactor according to the invention, the cross-sectional area of the reactor chamber increases gradually, in particular continuously. A gradual increase in the cross-sectional area of this type can, according to the invention, be achieved particularly advantageously if the reactor chamber is conical in shape. A gradual increase in the cross-sectional area enables the drops to be automatically sorted by diameter. Depending on the range over which the cross-sectional area of the reactor chamber increases and, if appropriate, the entry velocity of the gas stream, it is then possible for the atomized drops to have a greater particle size distribution as the range of increase of the cross-sectional area rises. The atomized drops can, as it were, be sorted by size and/or by weight. If the size and/or weight of the atomized drops increases, for example as a result of coalescence of drops, the larger and heavier drop will be able to remain suspended in the reactor chamber at a low height. The conical shape of the reactor chamber offers advantages both in terms of production engineering and in terms of flow dynamics. However, it is also quite conceivable that, depending on the task which the mist reactor is required to perform, to design a specific profile of the horizontal cross-sectional area of the reactor chamber. However, a round or possibly oval peripheral shape of the horizontal cross-sectional area will still be preferred, for reasons of flow dynamics. Tests carried out by the applicant have shown that it is advantageous if the apex angle of the conicity is at most 10°, preferably at most approximately 8°. Furthermore, tests have proven to the Applicant that it is advantageous if the apex angle of the conicity is at least 4°. This is to do with the Reynolds numbers, at which the gas stream passing through a conically widened passage becomes detached from the wall or starts to become turbulent. In this context, for information about the relationships between the Reynolds number and the apex angle, it is possible to refer to reference books, such as "Technische Strδmungslehre, Band 2: Anwendungen, by Brunno Eck, 1981, Springer-Verlag, Berlin", which on page 4 gives a table with recommended maximum apex angles for cones as a function of the Reynolds number of the gas flowing through them. Therefore, according to the invention it is particularly advantageous if, from the bottom upwards, the profile of the horizontal cross-sectional area is such that, at a predetermined entry velocity of the gas stream into the reactor chamber, the gas stream does not become detached from the wall of the reactor chamber or become turbulent.

If, from the bottom upwards, the horizontal cross-sectional area of the reactor chamber increases, there will also be an increasing risk of atomized droplets coming into contact with the wall of the reactor chamber as they drop downwards. According to the invention, this can be advantageously counteracted, if not prevented altogether, by providing the mist reactor with charging means for applying electric charge of identical polarity to, firstly, the mist and, secondly, the wall of the reactor chamber. Atomized droplets will then be repelled by the wall of the reactor chamber (since they both have the same charge polarity, positive or negative). It should be noted that it is possible to provide first charging means for applying an electric charge to the mist and to provide second charging means for applying an electric charge to the wall of the reactor chamber. However, it is also possible to use joint charging means for this purpose. Furthermore, it should be noted that the charging means for applying electric charge to the mist do not necessarily have to apply this charge directly to the mist, but rather it is also conceivable for the liquid or suspension to have been electrically charged by the first or joint charging means before being atomized.

To prevent atomized drops being carried out of the reactor chamber with the gas stream, it is not per se unusual to provide a demister in the outlet, which traps the atomized drops. If an electric charge has been applied to the mist, according to the invention the action of the demister can be improved considerably if the demister is provided with further charging means, for example third charging means, for applying an electric charge with a polarity which is opposite to that of the mist to the demister. In this way, improved collection of drops is achieved as a result of electrical attraction between the demister and the drop. In connection with the matter of fitting a mist reactor with charging means for applying electric charge of identical polarity to, firstly, the mist and, secondly, the wall of the reactor chamber and also, optionally, providing further charging means for applying an electric charge of opposite polarity to a demister, it should be noted that this can also be used in a mist reactor of countercurrent type in the general sense, i.e. either a vertically or horizontally arranged type or a type arranged in some other way, as a second aspect of the invention which is separate from the first aspect as described in Claim 1. An independent main claim relating to this second aspect can be worded as follows: Mist reactor, comprising a reactor chamber; atomization means arranged in the reactor chamber for atomizing a liquid or suspension to form a mist in the reactor chamber; an inlet, which opens out into the reactor chamber, for feeding a gas stream which is to be brought into contact with the mist and is to be passed through the reactor chamber; and an outlet, which opens out into the reactor chamber, for discharging the gas stream which has been passed through the reactor chamber, characterized in that the mist reactor also comprises charging means for applying electric charge of identical polarity to, firstly, the mist and, secondly, the wall of the reactor chamber. A subclaim which follows this main claim can then read:

Mist reactor according to the preceding claim, characterized in that a demister is provided in the outlet, and in that this demister is provided with further charging means for applying an electric charge with a polarity which is opposite to that of the mist to the demister. A mist reactor of this type according to the second aspect of the invention does not necessarily have to be of countercurrent type and also does not necessarily have to be arranged vertically. According to a preferred embodiment, a mist reactor of this type according to the second aspect will be of the countercurrent type and/or the vertically arranged type. Further particular embodiments of the second aspect will also emerge from the description of the figures. Furthermore, the characteristics of Claims 1-10 can be regarded as particular embodiments of the second aspect.

In order to prevent contact between the atomized drops and the reactor wall, it is possible, instead of the second aspect or in addition to the second aspect, according to a third aspect of the invention which, in the same way as the second aspect, can be regarded as completely separate from the first aspect but may also be considered in combination, therewith, for the wall of the reactor chamber to be of hydrophobic design, for example by coating it with Teflon. In this case, the drops adhere less well to the wall and can come off the wall again as a result of the force applied by the gas stream.

The present invention also relates to the use of a mist reactor according to the first and/or second and/or third aspect of the invention for the biological cleaning of gases.

The present invention also relates to the use of a mist reactor according to the first and/or second and/or third aspect of the invention as a countercurrent gas scrubber. The present invention also relates to the use of a mist reactor according to the first and/or second and/or third aspect of the invention as a gas dryer, which can be regarded as a particular form of gas scrubbing. H₂O can be removed from the gas by atomizing hydrophilic liquid or suspension, such as a concentrated solution of LiCl. The invention will be explained in more detail below on the basis of an exemplary embodiment which is diagrammatically depicted in the drawing. The drawing shows a mist reactor 1 having a reactor chamber 2. The reactor chamber 2 extends from a gas inlet 6, which is provided with a grid for uniformly distributing the gas, to atomization means 4, above which the outlet 5 for the gas is situated. In the outlet 5, there is a demister 10 for trapping atomized drops. The atomization means 4 may be designed in numerous ways. In the drawing, the atomization means 4 are diagrammatically depicted as a spray tube with a large number of spray nozzles. At the bottom of the mist reactor there is a so-called pressure chamber 9, to which, via a line 7 and a fan or possibly compressor 8, waste gases are supplied, so that they can flow out of the pressure chamber into the reactor chamber 2 via the grate 6 provided in the inlet. In the bottom of the pressure chamber 9 there is a bath containing a liquid or suspension, such as a suspension of micro-organisms. The liquid or suspension 11 is fed to the atomization means 4 via a line 12, pump 14 and line 13. A feed line 15 with a pump 16 also emerges at the bath 11 containing liquid or suspension, so that the bath can be supplied with liquid and/or suspension and/or additives. Furthermore, a discharge line 18 provided with a pump 17 for discharging waste water from the bath 11 opens out at the bath.

The reactor chamber is of conical design, with a conicity angle α of approximately 7°. The reactor chamber 2 has an axis of symmetry 22 which extends vertically. From the inlet 6 to the atomization means 4 or outlet 5, the horizontal cross- sectional area - the cross-sectional area perpendicular to the axis 22 - therefore increases gradually. This horizontal cross-sectional area also has a round peripheral contour.

If the flow velocity of the gas flowing through the reactor chamber 2 in direction 23 is selected to be sufficiently high, and if the drop size of mist which is atomized by the atomization means 4 is selected appropriately, the atomized drops will be able to remain suspended in the reactor chamber 2; as a result of the increase in the horizontal cross-sectional area of the reactor 2 in the vertical, upward direction, the drops with a smaller size will remain at a higher location - i.e. closer to the atomization means 4 - in the reactor chamber 2 than the larger and/or heavier drops, which will therefore remain suspended at a lower location - i.e. further away from the atomization means 4 - in the reactor chamber 2. In this way, the liquid hold-up in the reactor chamber 2 can be improved considerably. When the atomized drops ultimately reach the bottom of the reactor chamber 2, for example as a result of them becoming too large and/or heavy as a result of coalescence, because they have flowed downwards along the wall or because the flow velocity of the gas has been temporarily reduced, the mist will collect in the bath 11 via the pressure chamber 9, so that it can then be discharged or returned to the atomization means 4 via line 12, pump 14 and line 13.

To reduce the risk of the atomized drops coming into contact with the wall 3 of the reactor chamber 2, or even to eliminate this risk altogether, according to the invention it is advantageous for the wall 3 of the reactor chamber 2 and the atomized drops to be charged with an identical electrical polarity. According to the invention, this can be achieved by providing charging means. If the atomization means 4 are electrically conductive and are connected in an electrically conductive manner to an electrically conductive wall of the reactor chamber 2, this can easily be achieved by, as diagrammatically indicated at 20, the positive or negative pole, as desired, of a DC voltage source being connected to the wall of the reactor chamber 3. However, it will be clear that it is also possible for the reactor chamber 3 and the atomization means 4 each to be connected to a dedicated DC voltage source, provided that they are both connected to the positive or negative pole. As a result of the wall 3 of the reactor chamber 2 and the mist produced by the atomization means 4 being charged with an identical polarity, the atomized drops will be repelled by the wall 3, so that it will be less easy if not impossible for them to come into contact with this wall. In an embodiment of this type, to improve the drop-trapping action of the demister, it is possible to apply an electric charge of opposite polarity (to that of the atomized drops and wall 3) to the demister 10. This can be achieved, for example, by charging means 19 in the form of, for example, a DC voltage source, of which either the positive pole is connected to the demister 10, if the negative pole of the DC voltage source 20 is connected to the wall 3, or the negative pole is connected to the demister 10 if the positive pole of the DC voltage source 20 is connected to the wall 3. As indicated by dashed lines and reference numeral 21, in this case it is also possible, however, to make do with just one DC voltage source, of which one pole is connected to the wall 3 of the reactor chamber and the other pole is connected to the demister 10.