WO1992008544A1

WO1992008544A1 - An improved process for carrying out a photocatalytic reaction

Info

Publication number: WO1992008544A1
Application number: PCT/GB1991/001987
Authority: WO
Inventors: Po Lok Yue; Ali Mohammed Ali
Original assignee: The University Of Bath
Priority date: 1990-11-12
Filing date: 1991-11-12
Publication date: 1992-05-29
Also published as: AU8870991A; GB9024564D0

Abstract

In an improved process a photocatalytic reaction is carried out in a reaction vessel (1) having an associated light source (8) and containing a fluid reaction mixture and a mobile particulate catalyst, the reaction mixture being caused to circulate in the vessel in a swirl flow while being exposed to light from the light source by introducing a fluid into the vessel in a direction which has a major component tangential to the swirl flow. The process operates in continuous flow mode with the swirl flow ensuring effective mixing of the reaction mixture with the photocatalyst, and effective activation of the latter by the light source (8).

Description

An Improved Process for Carrying out a Photocatalytic Reaction

The invention relates to a process for carrying out a photocatalytic reaction and to a reaction vessel for carrying out such a reaction. The invention is particularly, but not exclusively, concerned with the photodegradation of organic compounds, especially the removal of organic pollutants from water by photodegradation. In recent years a need has arisen for water purifi¬ cation processes for degrading organic pollutants. Two types of photon-induced reactions have been found to be particularly effective. The first type involves ultra¬ violet/ozone photolysis and an example of one such reaction is described in U.S. Patent Specification

No. 4,230,571; the second type of reaction requires the presence ot a photocatalyst. In the latter processes, the contaminated water (which may be, for example, sewage, industrial or agricultural effluent, or merely drinking water) is irradiated by visible light or near ultra-violet light in the presence of a photocatalyst comprising an inorganic semiconductor solid such as Ti0₂. Photons possessing energies of the appropriate 'band-gap' mag¬ nitude are able to produce conduction band electrons and valence band holes in the solid, which can migrate to the liquid/solid interface to participate in charge transfer processes. In the presence of water or OH ions, highly reactive hydroxyl radicals are produced that are extremely effective in degrading the organic pollutants. In such processes the photocatalyst has either been immobilised on a stationary support or has been in the form of a particulate suspension in the water.

United States Patent Specification No. 4,892,713, for example, discloses a process in which the photocatalyst comprises a Ti0₂ coating on a fibreglass mesh sleeve. The sleeve is placed in a reaction vessel surrounding a cylindrical lamp and the contaminated water passed through the mesh sleeve, parallel with its length. That arrangement, however, has the disadvantage that utilisation of the photon energy by the immobilised catalyst is inevitably inefficient: photons emitted from the lamp are not able to penetrate deep into the fibre- glass mesh, with the result that an undesirably high proportion of the catalyst remains unactivated. Further¬ more, the degree of catalytic activity is limited by the surface area of the immobilised catalyst that is available for contact.

Processes in which the photocatalyst is employed in a powdered form are disclosed, for example, in United States Patent Specifications Nos. 4,861,484 and 4,863,608, respectively. In both processes, mechanical stirring means are required to achieve intimate mixing of the catalyst with the fluid medium. Although those processes could be operated on a small scale, it is unlikely that they would be economically viable for large scale water purification due to the cost of operating the stirring means. Furthermore, those processes are operated in the batch mode, wherein no decontaminated effluent may be drawn from the reaction vessel during the 5. reaction, which typically is of the order of at least an hour.

It is an object of the present invention to provide an improved process for carrying out a photocatalytic reaction and a reaction vessel for use therein, which 0 process overcomes or mitigates the above-mentioned disadvantages of the prior art.

The present invention provides a process in which a photocatalytic reaction is carried out in a reaction vessel having an associated light source and containing a fluid reaction mixture and a mobile particulate catalyst, the reaction mixture being caused to circulate in the vessel in a swirl flow while being exposed to light from the light source by introducing a fluid into the vessel in a direction which has a major component tangential to the swirl flow. The circulation of the reaction mixture is caused by the fluid introduced into the vessel and the swirl flow may be caused or enhanced by appropriate shaping of the vessel and/or the provision of internal baffles within the vessel. Thus, the present invention provides a process in which a mobile particulate photocatalyst may be very effectively mixed with a reaction mixture, without the use of mechanical stirring means, which process has the _ _

advantage that it operates in a continuous flow mode.

The term "swirl flow" is used to designate a circulation of fluid around an axis; the fluid may circulate helically along the axis or substantially in a single plane.

The vessel is preferably a substantially cylindrical vessel of substantially circular cross-section, with the axis of the swirl flow coincident with the axis of the vessel. In a preferred arrangement the fluid is introduced into the vessel in a direction which is aligned approximately tangentially with the swirl flow. Prefer¬ ably, the angle subtended between the said direction of the introduced fluid and the radius of the vessel that intersects that direction at the boundary of the vessel is about 100°.

The fluid may be introduced through a single inlet or a pair of inlet pipes arranged on opposite sides of the vessel. Two or more inlet pipes may be provided through which fluid is introduced at the same height. Usually, an outlet will be provided at one end of the vessel and the fluid will circulate in a helical swirl flow towards the outlet; in this way, it can be arranged for the fluid to have a predetermined residence time in the vessel. The outlet will preferably be aligned along the axis of the swirl flow. The vessel may be an upright cylindrical vessel with an outlet provided above the point where fluid is introduced into the vessel. ^— D —

Preferably the outlet is provided at the top of the vessel and the top of the vessel may be conical. An outlet pipe may project into the interior of the vessel; the pipe modifies and may improve the flow regime. Fluid may be introduced into the vessel at an angle of between 40° to 90°, and in particular 40° to 80°, to the vertical. Introduction of the fluid in a downwards direction helps to prevent disruption of the flow regime, and, in the case of a photocatalyst supported on larger, inert particles, helps to prevent such particles from leaving the reaction vessel; a preferred angle is about 60°.

Depending on the degree of degradation required and the organic compounds present it may be necessary to recycle the reaction mixture or use multiple vessels to obtain the desired result.

Although an arrangement in which the reaction mixture is irradiated by solar energy, for example, with the light source including reflective and/or focusing means, is not excluded, the reaction mixture will usually be irradiated by one or more artificial light sources. •These may be' placed around the outside of the reaction vessel and/or may be installed within the vessel, being immersed in the reaction mixture. In order to maximise activation of the catalyst, the swirl flow is preferably arranged to circulate about one or more light sources. The light source or light sources may themselves be positioned coaxially within the reaction vessel with the reaction mixture and the photocatalyst particles swirling around it/them. Such arrangements maximise the utilisa¬ tion of photons during the photocatalytic reaction, thereby enhancing the reaction rate. One particularly useful application of the inven¬ tion, as mentioned above, is a water purification process in which organic pollutants are removed from the water by photodegradation; such a process must necessarily be carried out on a large scale where the use of mechanical mixing means would be undesirable. For this application the photocatalyst may be any suitable semi-conductor material having one or more appropriate metals, such as, for example, Ti0₂, platinised Ti0₂, CdS, CdSe, Zn0₂, 0₃, or Sn0₂, although Ti0₂ is especially preferred. During this particular reaction, oxygen should also be fed into the reaction vessel, either mixed with the inlet stream(s) or independently.

The photocatalyst may be in a powdered form, for example, having a particle size ranging from a few nanometres to several millimetres, or, may be supported on an inert, particulate support such as, for example, glass beads, or ceramic, alumina or silica. The latter form is especially preferred because the larger particles of the support material may be selected to suit the dynamic requirements of the reaction vessel and for the minimisation of downstream separation problems; for example, the fluid may be introduced and removed from the vessel in such a manner that catalyst particles above a minimum size are retained in the vessel. The catalyst may be grafted onto the inert support by physical adhesion, precipitation, co-precipitation, impregnation, ion-exchange or chemical bonding. The photocatalyst particles may be formed by agglomeration, tableting, comminution or a sol gel process to obtain the desired particle size. The concentration of the particles in the reaction mixture in the vessel should be sufficient to ensure efficient catalysis, while not being so high as to prevent photons penetrating into the reaction vessel thereby impairing their utilisation.

The process variables include: flow rate of fluid, nature of catalyst, size or size distribution of catalyst particles, loading (i.e. concentration of catalyst particles in the reactor) of catalyst, oxygen flow rate, concentration of organics in the reactor and light intensity, the rate of destruction of organics being dependent on many of the variables listed above.

The process of the invention may be used very effec- tively to degrade a wide range of organic pollutants, such as, for example, the following generic classes of organic compounds:

(1) Organics usually found in industrial solvents e.g. di-chloroethaneε, tri- and per-chloroethylenes, alcohols, ketones and esters.

(2) Saturated and unsaturated aliphatic and aromatic organics e.g. alkanes, alkenes, benzene, toluene.

(3) Phenol, alkylphenols, catechols, cresols. (4) Brominated aliphatic organics, e.g. ethylene dibromide.

(5) Halogenated aromatic organics e.g. chlorinated phenols, chlorinated biphenylε, polychlorinated biphenyls and dioxins.

(6) Nitrogen containing organics e.g. nitrobenzene and alkylnitrobenzenes.

(7) Pesticides and herbicides, e.g. lindane, atrazine. The present invention also provides an apparatus for carrying out a photocatalytic reaction, the apparatus including a reaction vessel of substantially cylindrical shape for receiving a fluid reaction mixture and a mobile particulate catalyst, and a light source associated with the vessel, the vessel including one or more inlets in its side wall arranged to direct fluid into the vessel in a direction which has a major component tangential to the side wall of the vessel.

Reaction vessels constructed in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figs, la and lb are, respectively, schematic sectional and plan views of a first reaction vessel;

Figs. 2a and 2b are schematic sectional views of two further reaction vessels; Fig. 3 is a schematic sectional view showing the flow regime existing in the reaction vessel of Figs, la and lb (omitting those velocity components perpendicular to the page) ; Fig. 4 is a schematic perspective view of the flow regime existing in the reaction vessel of Figs, la and lb;

Fig. 5 is a graph showing the degradation with time of a pesticide;

Fig. 6 is a graph showing the degradation with time of chloroform; and,

Fig. 7 is a schematic diagram showing the arrangement of the apparatus for continuous mode operation.

The reaction vessel shown in Figs, la and lb is a prototype photoreactor for use in carrying out photo¬ catalytic reactions involving the degradation of organic pollutants. The reaction vessel or reactor 1 is mounted in an upright position and comprises a cylindrical outer wall 2 of circular cross-section. The top of the reactor is sealed by a cover 3 in which a tubular outlet 4 is centrally disposed. Two supply pipes 5a, 5b are verti- cally disposed one on either side of the reactor 1 (only one being shown in Fig. la). An upper inlet pipe 6 and a lower inlet pipe 7 lead from each supply pipe 5a, 5b into the reactor 1, so that one pair of inlet pipes are positioned near the bottom of the reactor and the other pair directly above, about halfway up the outer wall.

Each inlet pipe 6, 7 is inclined at an angle of about 60° to the vertical, and, as seen in Fig. lb, extends towards the outer wall 2 approximately tangentially in a clock- wise direction. The angle subtended between the axis of each pipe 6, 7 and the radius of the reactor 1 that intersects the pipe axis at the outlet of the pipe is about 100°. UV radiation of a suitable wavelength is provided by a tubular lamp 8 disposed co-axially within the reactor vessel. A jacket 9 is placed around the lamp 8 and supplied with coolant in order to minimise heating.

During operation, contaminated water containing the organic pollutants is continually pumped, at an appropriate volumetric rate, into the reactor vessel 1, through the two pairs of tangential inlet pipes 6, 7. Oxygen is fed at a suitable rate into the reaction vessel 1, being mixed with the inlet stream. The arrangement of the pipes 6, 7 together with the selected flow rate, ensure that the water enters the reactor 1 with such a momentum that the reaction mixture is caused to circulate in the clockwise direction around the lamp 8 in an upward helical "swirl flow pattern", as shown in Figs. 3 and 4. The water then exits the vessel 1 through the outlet 4. Fig. 3 shows the flow pattern in a vertical plane through the central axis of the vessel and shows only the component of motion within the plane. Fig. 4, on the other hand, shows the upward flow along a helical path of the mixture as it passes towards the outlet 4.

The swirl pattern ensures that the catalyst particles mix with the water in an extremely efficient manner. Moreover, in addition to the mixing being enhanced as compared with conventional mechanical agitation methods, the utilisation of photons is also more efficient because the catalyst particles are caused to circulate around the immersed light source 8.

The reaction vessel 1 forms part of apparatus arranged to operate .in a continuous flow mode, with or without recycling of some or all of the reaction mixture, as shown in Fig. 7. The contaminated water is pumped from a circulation tank 10 via pump 11 and flow meter 12 to the reactor 1 at a suitably low predetermined flow rate. After a predetermined residence time, that water then exits the vessel 1 through the outlet 4, whereupon it can either be removed entirely from the system or some or all of it can be pumped back to the circulation tank 10 for recycling.

Two further reaction vessels are shown schematically in Figs. 2a and 2b. Each vessel is similar to that shown in Figs, la and lb but has been modified so that the central outlet pipe 4 extends downwardly about halfway into the interior of the vessel. The presence of the exit pipe projecting into the top of the vessel may improve the circulation pattern within the vessel, and causes the reaction mixture to be brought into close proximity with the light source (not shown, but disposed coaxially within the exit pipe) as it leaves the vessel.

The vessel shown in Fig. 2a is of a similar shape to that in Fig. 1 but the vessel shown in Fig. 2b is provided with a conical top portion 20 so as to promote the desired pattern of flow of liquid within the vessel. Such a conical end could also be provided on the vessel shown in Figs, la and lb. It should be appreciated that the apparatus described above may be modified to suit particular requirements. Thus, instead of employing a cylindrical reaction vessel within which a single swirl flow is established, a larger vessel may be used in which a plurality of swirl flows, each preferably centred about a respective light source, are established. A reaction mixture entering the vessel may be caused to pass through each of the swirl flows before leaving the vessel (so that the swirl flows may be regarded as being arranged in series) or to pass through only one or some of the swirl .flows with another portion of the reaction mixture being caused to pass through other swirl flows (so that the swirl flows may be regarded as arranged in parallel). In either case the shape of the vessel may be selected and internal baffles may also be provided to promote the desired flow pattern. The inlets and outlets may be sited at the boundary wall of jhe vessel or one or more pipes may extend within the interior of the vessel so as to provide an inlet or outlet within the interior of the vessel; for example, respective inlets and/or outlets may be provided around each light source to establish an associated swirl flow therearound. Examples of photocatalytic reactions carried out in accordance with the invention are described below. Examples

A reaction vessel, as shown in Figs, la and lb above, was employed for the reactions. The vessel was provided with two pairs of tangential inlet pipes located at heights of 8 cm and 40 cm, respectively, the vessel itself having a height of 1 m and a diameter of 0.4 m. Each inlet pipe had a diameter of 2.5 cm. A 40 black lamp was immersed in the reaction mixture in the vessel. Oxygen was supplied separately to the reactor vessel at a volumetric rate of ll/min.

Degussa P25 Ti0₂ powdered catalyst, ranging in particle size from 10 nm to 100 nm, was present in the reaction mixture in a concentration of about 1 g/1. The water to be treated was prepared by the addition to it of an amount of the pesticide lindane, the amount being sufficient to produce, in the reaction vessel, a concentration of 1.5 mg/1. ■ The reaction vessel was set up for operation in the recycling mode, whereby the entire effluent stream was recycled back to the inlets. The contaminated water was pumped into the vessel at a flow rate of 20 1/min.

The reaction was carried out over a total period of time of 6 hours, during which the reaction mixture was constantly illuminated by the UV lamp.

The degradation of the pesticide with total circula¬ tion time is shown by means ^'of a graph i Fig. 5. It will be seen that the pesticide, despite being an extremely stable substance, was quite effectively degraded. Although the rate of degradation appears to be rather slow, it should be noted that the horizontal axis shows the total circulation time in minutes through the recycle loop, whereas the actual residence time of the fluid spent in the reactor (i.e. the irradiation time) is less than one-third of the circulation time. Furthermore it should be noted that the rate of degradation may be easily increased by increasing the power of the light source and/or the catalyst concentration (providing there is no overloading) .

The results of a similar photodegradation reaction in which chloroform was degraded are shown in Fig. 6 as a graph of total organic carbon (TOC) concentration against total circulation time. In this reaction, the rate of degradation was much more rapid with hardly any of the organic pollutant remaining after about 15 minutes of circulation time (equivalent to less than 5 minutes of irradiation time) .

Claims

Claims :

1. A process in which a photocatalytic reaction is carried out in a reaction vessel having an associated light source and containing a fluid reaction mixture and a mobile particulate catalyst, the reaction mixture being caused to circulate in the vessel in a swirl flow while being exposed to light from the light source by introducing a fluid into the vessel in a direction which has a major component tangential to the swirl flow.

2. A process as claimed in claim 1, wherein the vessel is a substantially cylindrical vessel of substan¬ tially circular cross-section with the axis of the swirl flow coincident with the axis of the vessel.

3. A process as claimed in claim 1 or claim 2, wherein the fluid is introduced into the vessel in a direction which is aligned approximately tangentially with the swirl flow.

4. A process as claimed in claim 3, wherein the angle subtended between the said direction of the introduced fluid and the radius of the vessel that inter¬ sects that direction at the boundary of the vessel is about 100°.

5. A process as claimed in any one of claims 1 to 4, wherein the fluid is introduced through a pair of inlet pipes arranged on opposite sides of the vessel.

6. A process as claimed in any of claims 1 to 5, wherein an outlet is provided at one end of the vessel.

7. A process as claimed in any of claims 1 to 6, wherein two or more inlet pipes through which fluid is introduced are provided at the same height.

8. A process as claimed in any of claims 1 to 7, wherein the vessel is an upright cylindrical vessel with an outlet provided above the point where fluid is introduced into the vessel.

9. A process as claimed in any of claims 1 to 8, wherein an outlet pipe is provided at the end of the vessel and projects into the interior thereof.

10. A process as claimed in any of claims 1 to 9 , wherein fluid is introduced into the vessel at an angle of between 40° and 80° to the vertical.

11. A process as claimed in claim 10 , wherein the angle is about 60°.

12. A process as claimed in any of the above claims, wherein the reaction mixture is irradiated by one or more artificial light sources.

13. A process as claimed in claim 12, wherein at least one light source is positioned co-axially within the reaction vessel on the central axis about which the reaction mixture swirls.

14. A process as claimed in any of the above claims, wherein the photocatalyst is in a powdered form.

15. A process as claimed in any of claims 1 to 13, wherein the photocatalyst is supported on an inert particulate support.

16. A process as claimed in any of the above claims, wherein the process is a water purification process in which organic pollutants are removed from the water by photodegradation.

17. A process as claimed in claim 16, wherein the photocatalyst comprises Ti0₂.

18. A process substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

19. A process in which a photocatalytic reaction is carried out in a reaction vessel having an associated light source and containing a fluid reaction mixture and a mobile particulate catalyst, the reaction mixture and catalyst^' being caused, by fluid introduced into the vessel, to circulate within the vessel in a predetermined flow pattern which is such that fluid entering the vessel is brought repeatedly, together with the mobile catalyst, into close proximity with the light source.

20. A process as claimed in any one of claims 1 to 19, wherein the fluid is introduced and removed from the vessel in such a manner that catalyst particles above a minimum size are retained in the vessel.

21. An apparatus for carrying our a photocatalytic .reaction, the apparatus .including a reaction vessel of substantially cylindrical shape for receiving a fluid reaction mixture and a mobile particulate catalyst, and a light source associated with the vessel, the vessel including one or more inlets in its side wall arranged to direct fluid into the vessel in a direction which has a major component tangential to the side wall of the vessel.

22. An apparatus according to claim 21 for carrying out a process according to any of claims 1 to 20.

23. An apparatus for carrying out a photocatalytic reaction, the apparatus being substantially as herein¬ before described with reference to, and as shown in, Figs, la, lb, 2a, 2b, 3 or 4 of the accompanying drawings.