WO2018011550A1

WO2018011550A1 - Photochemical reactor

Info

Publication number: WO2018011550A1
Application number: PCT/GB2017/052014
Authority: WO
Inventors: Kevin BOOKER-MILBURN; Luke ELLIOTT
Original assignee: University of Bristol
Current assignee: University of Bristol
Priority date: 2016-07-11
Filing date: 2017-07-07
Publication date: 2018-01-18
Anticipated expiration: 2019-01-11
Also published as: GB2552171A; GB2552171B; GB201612034D0

Abstract

A photoreactor is described, the reactor including a first end piece, a second end piece, a plurality of elongate reactor tubes extending between the first end piece and the second end piece, the reactor tubes being axially distributed so as to define a lamp receiving recess, and two or more fluid connectors connecting at least three of the reactor tubes. Usually, the reactor will include a coolant jacket surrounding the reactor tubes, the coolant jacket allowing circulating coolant to flow around and over the reactor tubes. Also described in a photochemical process using the photoreactor. The photoreactor may be used with high power lamps.

Description

Photochemical Reactor

The present invention relates to photochemical reactors and to photochemical processes conducted using such reactors. Reactors for use in chemical reactions are of various types.

WO-A-2014/200975 discloses a solar thermochemical reactor which uses solar power to facilitate thermochemical reactions. The thermochemical reactor comprises an aperture for receiving solar radiation, the aperture being disposed in a reactor member, a plurality of independent absorber tubes, and a reactive material, the reactive material being disposed in the plurality of absorber tubes.

Photochemical reactions have been used in waste water treatment and in photochemical synthesis. Photochemical reactions are usually performed using purpose built photochemical reactors.

Natarajan et al Chem. Eng. Journal (2011) 178 pp. 40 to 49 describe a simple system for photocatalytic treatment of water using titania coated quartz tubes and small light emitting diode (LED) light sources.

Batch photochemical reactors are known and have been used for many years.

CN204193929U discloses a normal pressure visible photocatalytic parallel reaction instrument for batch processing comprising a base and a cover plate and a plurality of reaction tube holes for placing reaction tubes formed in the cover plate. The instrument is capable of performing a plurality of independent, parallel reactions simultaneously.

CN201625528U discloses a photocatalytic reactor for decomposing organic pollutants, the photocatalytic reactor comprising a reactor outer casing, an upper seal cover, a lower seal cover, a water inlet, a water outlet, an ultraviolet light tube and glass tubes wound on the periphery of the ultraviolet light tube. The surfaces of the glass tubes have titanium dioxide coating films and act as substrates for the photochemical reaction as organic contaminants in water flow through the annular cavity in which the glass tubes are situated.

US-B-5,093,086 discloses an improved photochemical reactor useful for the isotopic enrichment of a predetermined isotope of mercury, especially ¹⁹⁶Hg. The reactor has cooling fluid in contact with the envelope of the lamp and uses separate straight tubes and randomly oriented pieces of tubing to improve the enrichment factor and utilization compared to a non-packed reactor.

Tymoschuk et al. (Ind. Eng. Chem. Res. 1993, 32, 1342 - 1353) disclose modelling and verification studies of a photoreactor using an experimental setup of a single tube and low power lamps which required no active cooling.

Recently, the use of continuous or flow reactors in organic synthesis has become more common. A number of flow reactors have been developed for both research and production for organic, photochemical reactions. Flow reactors offer advantages compared to batch reactors in heat and mass transfer, efficient mixing, scale-up, safety and, often, cost.

A practical flow reactor for photochemical synthesis on scales of up to a few hundred grams per day is described by Booker-Milburn et al, J. Org. Chem. (2005) 70, pp. 7558 to 7564. The reactor consists of a single length of ultraviolet (UV) transparent fluorinated ethylene propylene (FEP) tubing wrapped closely around a UV source. One, two or three layers of FEP may be coiled around the UV source to give a reactor of high surface area and excellent UV capture.

FEP coil reactors are also disclosed in Blanco-Ania et al, Org. Process Research & Devel. (DOI 10.1021/acs.oprd.5b00354). (2016) 20, pp. 409 to 413.

Coiled FEP tubing is reasonably strong mechanically. However, kinks or abrasions in the tubing may lead to weak spots that can rupture under pressure requiring the whole length of FEP tubing to be replaced.

Quartz coil reactors where a coil of quartz tubing is wrapped around a UV light source are described in Shen et al, Chem. Commun. (2012) 48, pp. 7444 to 7446, together with an aluminium mirror to enhance efficiency. Quartz coil reactors do not suffer from the kinking problem. However, quartz may be fragile in coils and is expensive to form into coils.

There is a need for a high capacity photochemical reactor to produce products on the kg/day scale. Such reactors are likely to require high power light sources and therefore to generate large amounts of heat. Such reactors should make good use of the available light, avoid problems resulting from over-heating and capture as much light as possible both for efficiency and safety (especially if the light source is high- power UV). The reactor should be relatively low cost and have a relatively small footprint for convenience in both research and production and be such that field repairs are possible and convenient.

It is an aim of the present invention to provide an improved reactor which addresses these needs and mitigates or avoids problems with known reactors.

In a first aspect, the present invention accordingly provides a photoreactor comprising, a first end piece, a second end piece, a plurality of elongate reactor tubes extending between the first end piece and the second end piece, the reactor tubes being axially distributed so as to define a lamp receiving recess, and a plurality of fluid connectors connecting at least some of the reactor tubes, preferably two or more fluid connectors connecting at least three of the reactor tubes.

This is greatly advantageous, because such a photoreactor makes good use of the available light (capturing a great deal of light) leading to high efficiency and improving safety. The photoreactor provides a small footprint and, by replacing easily obtainable reactor tubes, field repairs are straightforward. Because at least some of the reactor tubes are provided so as to be in fluid connection by use of fluid connectors, reactant solutions may flow through one reactor tube, through the fluid connector, and into another reactor tube thus increasing very significantly the residence time.

In use, reactant mixture/solution flows through the reactor tubes. The photoreactor may be used with separate flows of reactant solution in separate parts of the reactor, each separate flow of reactant solution flowing through a reactor tube from a reactor inlet through a fluid connector to at least a second reactor tube, optionally, through one or more further fluid connectors and reactor tubes and then to a reactor outlet. Each reactant solution may be the same or different. Such an arrangement would be a suitable for relatively low flow rates and/or for

photochemical reactions which require relatively low residence times.

Usually, each reactor tube will be connected by a fluid connector so as to be in fluid connection with at least one other reactor tube, preferably two other reactor tubes. Preferably, the reactor tubes are connected by fluid connectors so that fluid flowing through a first reactor tube flows in a first direction and flows into an adjacent reactor tube in a second, preferably, the opposite, direction.

In the more preferred embodiments, all the reactor tubes are connected by fluid connectors so all the reactor tubes are in fluid connection. This provides longer residence time.

The photoreactor may further comprise a first end cap and a second end cap.

The fluid connector may be a length of tubing. However, conveniently the fluid connectors may be comprised/formed in the first end piece and/or the second end piece and/or the fluid connectors may be comprised/formed in the first end cap and/or the second end cap. The fluid connectors (or at least a part of the fluid connector) may take the form of grooves adapted to sealingly receive the ends of at least two reactor tubes.

Preferably, the photochemical reactor further comprises a coolant jacket surrounding the reactor tubes, the coolant jacket allowing circulating coolant to flow around and over the reactor tubes, in use. This is advantageous because using circulating/flowing cooling fluid/coolant fully surrounding and encapsulated the reactor tubes allows the use of very high powered photochemical lamps without overheating. Thus, in a second aspect, the present invention provides a photoreactor comprising, a first end piece, a second end piece, a plurality of elongate reactor tubes extending between the first end piece and the second end piece, the reactor tubes being axially distributed so as to define a lamp receiving recess, a plurality of fluid connectors connecting at least some of the reactor tubes, preferably two or more fluid connectors connecting at least three of the reactor tubes, and a coolant jacket surrounding the reactor tubes, the coolant jacket allowing circulating coolant to flow around and over the reactor tubes, in use.

The photoreactor may conveniently further comprise an inner tube situated in the lamp receiving recess, preferably an inner tube transparent to light of the wavelength to be used in the photoreactor.

For safety and protection of the components, it is preferred that the photoreactor further comprises an outer tube surrounding the reactor tubes. The outer tube may be metallic.

Advantageously, the inner tube and outer tube may be sealingly connected to the first end piece and second end piece, thereby forming a vessel containing the reactor tubes, preferably thereby forming the coolant jacket containing the reactor tubes.

The coolant jacket, in use, would have circulating coolant flowing through an inlet, circulating, flowing around and fully surrounding the reactor tubes and out through an outlet (with or without recirculation). The coolant/cooling fluid may be, for example, water or a mixture of water/glycol.

Preferably, the outer tube comprises a fluid/coolant inlet and a fluid/coolant outlet, which may be used to circulate cooling/coolant fluid, usually liquid. If the inner and outer tubes are cylindrical, the vessel/coolant jacket would have a generally annular cross-section.

Usually, the photoreactor will comprise at least a first reactor inlet and at least a first reactor outlet. Preferably, the photochemical reactor comprises at least a first reactor inlet connected to a first reactor tube and at least a first reactor outlet connected to a second reactor tube. Each reactor tube may be substantially straight, which is advantageous because straight tubes are generally easier to keep clean and may be less delicate and expensive (e.g. if made of quartz) than curved reactor tubes. The reactor tubes in the photoreactor are preferably substantially parallel. It is advantageous if the reactor tubes are such as to ensure adequate mixing of the reactant solution as it flows through each reactor tube. Thus, the diameter of the reactor tubes is preferably selected so as to ensure, for the reaction solution and flow rates to be used, turbulent flow. A, or each, reactor tube may comprise a static mixer (or active mixer) to provide good mixing of the reactant solution.

Each reactor tube may have a diameter (e.g. internal diameter) suitable for the application of the photochemical reactor. If relatively high flow rates are to be used the internal diameter of each reactor tube may generally be higher than if relatively low flow rates are to be used. Thus, the internal diameter of each reactor tube may be in the range 0.5 mm to 10 mm, preferably 1-3 mm.

Preferably, the reactor tubes are distributed in at least one concentric array.

In order to capture more of the light from the lamp, the reactor tubes are preferably distributed in an inner concentric array and an outer concentric array. Preferably, each reactor tube in the outer concentric array is offset with respect to a reactor tube in the inner concentric array. There may be two concentric arrays, three concentric arrays, four concentric arrays or more concentric arrays. In each case the reactor tubes of a concentric array are preferably offset with respect to the reactor tubes of an adjacent array or arrays. To further increase capture of the light, the outer tube, if present, may comprise a reflective inner surface. The outer tube may be metallic (e.g. stainless steel or aluminium) and the inner surface may be polished or highly polished.

The inner tube may be made of generally any suitable material that is sufficiently transparent (e.g. having a light transmission of the appropriate wavelength above 80%) to light of the appropriate wavelength. Useful materials that the inner tube may comprise include quartz, Pyrex or Vycor. A quartz inner tube is useful if the wavelength range is about 200 nm to visible (i.e. up to around 800 nm), a Pyrex inner tube is useful if the wavelength range is 280 nm to visible, and a Vycor inner tube if the wavelength range is 250 nm to visible. To further modify the wavelength and/or intensity of light the photoreactor may comprise a light filter located between the lamp receiving recess and the reactor tubes. Such a filter may be conveniently changed for different purposes and different wavelengths. The filter may be a filter (e.g. a multilayer coating) on at least one surface of the inner tube or may be a separate filter, liquid filter or coolant.

Each reactor tube may be made of generally any suitable material that is sufficiently transparent (e.g. having a light transmission of the appropriate wavelength above 80%) to light of the appropriate wavelength and has the appropriate mechanical, and surface properties and if appropriate thermal resistance. Each reactor tube may comprise a polymer (e.g. a fluorinated polymer for example FEP), but preferably each reactor tube comprises quartz, Pyrex, Vycor or a ceramic.

Although, FEP is a versatile UV transmissive material, it is not completely transparent and adding more arrays in a larger reactor would likely result in significant loss of UV transmission. The inventors of the present invention have surprisingly found that degradation of the internal FEP surface and reduction in UV transmission can occur on long-term irradiation with shortwave UV. Certain reactions result in fouling of the FEP surface. Sometimes this can be reversed by a short flush of MeOH or DMSO, but on other occasions the damage is irreversible and the FEP tubing has to be completely replaced. The photoreactor preferably further comprises a cooling fan located so as to direct flow into the lamp receiving recess. The cooling fan may be located at the same end of the photoreactor as one or more of the fluid connectors, or may be at the other end of the photoreactor from one or more of the fluid connectors.

In use, the photoreactor will further comprise a lamp located in the lamp receiving cavity/recess. The lamp may be an ultraviolet lamp, for example the lamp may be a medium pressure Hg ultraviolet lamp. For other reactions, lamps generating light of other wavelengths may be used.

An advantage of the present invention is that high power lamps may be used. Thus, the lamp may be a 0.2 to 80 kW lamp, preferably a 0.2 to 60 kW lamp, more preferably a 0.2 to 10 kW lamp. Photoreactors according to the invention find use in many areas of

photochemistry including UV photochemistry and photocatalysis.

Thus, in a third aspect, the invention provides a photochemical process comprising providing at least a first reactant solution and flowing the first reactant solution through (the reactor tubes of) a photochemical reactor according to the first aspect whilst irradiating the first reactant solution with light of a predetermined wavelength.

The lamp will usually generate light over a predetermined range of

wavelengths. The predetermined wavelength may be in the range 200 nm to 800 nm. The light may be ultraviolet light having wavelength(s) generally in the range 220 nm to 400 nm, preferably 250 nm to 400 nm. The light may be visible light (useful for e.g. photocatalytic reactions) in the range 350 nm to 650 nm, preferably 400 nm to 625 nm. Particular wavelengths that may be useful include (e.g. with low pressure Hg lamps) 250 nm, 310 nm, 360nm and with e.g. light emitting diode (LED) lamps having visible light wavelengths in the range 400 nm and 600 nm.

The concentration of the reactant solution will usually depend on the particular reaction to be undertaken. The flow rate of reactant through the photoreactor may be 0.1 ml/min to 1 litre/min. For water purification uses of the reactor and photochemical process, the flow rate may be higher, for example 500 ml/min to 100 1/min. The present invention will now be described by way of example only, and with reference to, the accompanying drawings, in which:

Figure 1 illustrates a perspective view of a photoreactor according to an embodiment of the invention.

Figure 2 is an exploded view of the photoreactor of Figure 1. Figure 3 is a top plan view of the photoreactor of Figure 1.

Figure 4 is a bottom plan view of the photoreactor of Figure 1.

Figure 5 is a schematic cross section of a photoreactor according to the invention. In the Figures, the same reference numbers refer to the same part.

Figure 1 shows a perspective view of a photoreactor 2 according to the invention. The photoreactor 2 comprises a metallic outer tube 4 of stainless steel which protects the internal components of the photoreactor 2 and extends between a lower end cap 6 and an upper end cap 8. The end caps 6, 8 are formed of aluminium or poly ether ether ketone (PEEK) but may, alternatively, be formed of any other suitable material. The upper end cap 8 comprises a lamp receiving aperture 18 leading to the lamp receiving recess (not visible in Figure 1 : see Figure 5). A fan (not shown) may be mounted over the lamp receiving aperture to direct cooling gas flow inside the photoreactor 2 and in particular in the lamp receiving recess to cool the lamp when the lamp is inserted and in operation.

The fan may instead be mounted at the other end of the photoreactor at the lower end cap 6 and therefore at the other end of the photoreactor 2 from the reactor inlet 32, reactor outlet 34. The upper end cap 8 comprises fixing screws 14. A reactor inlet aperture 32a is for the reaction mixture to be introduced into the reactor inlet 32 and thus into the photoreactor 2. The reactor outlet aperture 34a is for the reaction mixture to be removed from photoreactor 2 via the reactor outlet 34. The outer tube 4 comprises a cooling fluid inlet 10 and cooling fluid outlet 12 for circulation of cooling fluid. The photoreactor of the invention, an embodiment of which is illustrated in the

Figures, has a design of a modular reactor with a series of parallel reactor tubes (of quartz) arranged axially and concentrically around a lamp recess for receiving a high power UV (or other wavelength) light source. By adopting a concentric arrangement of parallel reactor tubes the reactor can encapsulate light just as effectively as the previous used FEP reactors. The reactor may be surrounded by a (e.g. a metal) tube with a reflective inner surface, to make the use of available light even more efficient and to improve safety. Use of the reflective inner surface may, depending on the reaction, increase productivity by 15% to 30% or more.

At the centre of the concentric reactor array, in the lamp receiving recess, a standard low-cost, high power medium pressure Hg-lamp may be situated. In one arrangement, the reaction solution enters through a reactor inlet in an end-cap/end- piece flows through one tube, into a fluid connector in the other end-cap/end-piece, then back through the adjacent tube in the opposite direction. This may be repeated for all tubes in fluid connection in the reactor until the solution exits the reactor through a reactor outlet in an end cap. This arrangement enables a relatively thin profile of solution to receive a high light exposure by the time it had passed in a zigzag, counter flowing manner through the reactor assembly. Any light that passes through or between the tubes may be reflected back in close proximity to the reaction solution by the reflective inner surface of the outer tube. Figure 2 shows an exploded view of the photoreactor 2 illustrating the internal, components. The working part of the photoreactor 2 comprises a plurality of reactor tubes 28, 30 constructed of quartz extending between the lower end piece 23 and upper end piece 25, with the respective ends of the reactor tubes 28, 30 located within apertures in the lower end piece 23 and upper end piece 25. The reactor tubes 28, 30 are distributed around the axis of the photoreactor (i.e. are axially distributed) to form an inner recess/cavity (see Figure 5) for receiving the lamp (e.g. UV lamp, not shown in Figure 2). The reactor tubes 28, 30 are arranged in two concentric arrays: an inner concentric array of inner reactor tubes 28 and an outer concentric array of outer reactor tubes 30. Each outer reactor tube 30 is offset relative to the inner reactor tubes 28 to increase the amount of light captured. Although two concentric arrays of reactor tubes 28, 30 are shown in Figure 2, the photoreactor 2 may instead have a single concentric array of reactor tubes, or three, four, five or more concentric arrays of reactor tubes, if desired.

Lower spacer 22 and upper spacer 24 sit between the lower end piece 23 and lower end cap 6 and between the upper end piece 25 and upper end cap 8 respectively and help to protect, seal and maintain orientation of the reactor tubes 28, 30 at least by providing a compressive force on the seals of the reactor tubes and also to provide a flat surface for the connector groove O rings 36 to butt up to. The end caps 6, 8 each have a plurality of connector grooves 38 and connector groove O ring seals 36 (comprising EPDM or Kalrez™) to receive the end of two adjacent reactor tubes 28; 30 and fluidly connect those reactor tubes 28; 30. Each reactor tube 28; 30 is thereby in fluid connection with two other reactor tubes 28; 30 so that all the reactor tubes 28; 30 in the photoreactor 2 are in fluid connection. When reactant solution/mixture is introduced into the photoreactor 2 through reactor inlet 32 in the upper part of the photoreactor (as illustrated) using a flow pump (not shown), the reactant solution flows in a first direction through a reactor tube 28, through the respective connector groove 38 in the lower end cap 6 and into an adjacent reactor tube 28. The reactant solution flows through the adjacent reactor tube 28 in a second direction (which is opposite to the first direction) and then through a further connector groove (not visible in Figure 2) in the upper end cap 8 into a third reactor tube 28 and so on. After flowing through all the reactor tubes 28, 30 the reactant solution flows through the reactor outlet. The end caps 6, 8 each have fixing screws 14 to fix the respective end cap 6, 8 to the respective end piece 23, 25 and to provide a compressive force to maintain the integrity of the seals.

In alternative arrangements, all the reactor tubes may not be in fluid connection or only a portion of the reactor tubes may be in fluid connection. In such cases there may be two or more reactor inlets and two or more reactor outlets. Such arrangements may be advantageous for reactions that may require low residence time and/or if two or more different reactions are to be performed at the same time.

In use, inner tube 20 of quartz (or another material suitable for the wavelength to be used in the reactor e.g. of Pyrex™ or Vycor™) is inserted inside the lamp recess/cavity and, when the photoreactor 2 is fully assembled, sealingly connects to the upper and lower end pieces 23 and 25. When the metallic outer tube 4 is also installed to surround the reactor tubes 28, 30 and also sealingly connected to the upper and lower end pieces 23, 25 the inner tube 20 and outer tube 4 cooperate to define a vessel of generally annular cross section surrounding the reactor tubes 28, 30. The vessel may act as a cooling jacket, with cooling fluid circulated from cooling fluid inlet 10 to cooling fluid outlet 12 to cool the reactor tubes 28, 30 during operation. The cooling fluid may be water (e.g. from the water mains) or a circulating chiller liquid (e.g. water/glycol). The cooling fluid may also act as a light filter. The inner tube and outer tube are preferably only sealingly connected to upper and lower end pieces 23 and 25. The end cap and spacers may then be removed without breaking the seal of the inner and outer tubes.

The material of the inner tube 20 may be selected depending on the wavelength of light to be used in the photoreactor 2 and the inner tube 20 may conveniently be changed as desired. Furthermore, the inner tube 20 may act as a light filter to pass predetermined wavelengths of light or an additional filter may be deposited as a coating (e.g. a multilayer coating) on a surface of the inner tube and/or a filter may be inserted between the lamp and inner tube 20 or between the inner tube 20 and reactor tubes 28, 30. A fan as discussed with reference to Figure 1, above, may direct cooling flow (usually of air) over the lamp during operation. A combination of liquid cooling using the cooling jacket formed by the inner tube 20 and outer tube 4 and cooling flow from a fan enables the photoreactor to operate without excessive temperature increase even when the lamp is of high power (e.g. 5 kW or higher) since the reactor tubes would be encapsulated within a fluid-cooled annular cavity so that any irradiated IR reaching the tubes would be quickly absorbed by the circulating coolant. In addition, a fan installed at one end of the reactor may remove stagnant hot air (and any ozone produced by the lamp), thus preventing overheating, in particular when the assembly is enclosed in the outer tube 4. This dual mode of cooling is effective at negating the heating effects produced by the very powerful UV source. This also has the advantage that a lamp would not itself require to be contained within a cooling jacket, thus simplifying the design.

The outer tube 4 may have a reflective inner surface to ensure efficient use of light from the lamp in the event there is any light leakage between the array(s) of reactor tubes. Thus, the design of the photoreactor maximises light (e.g. UV) capture because of the arrangement of inner and outer sets of concentric quartz tubes with the outer array offset to cover the space between the tubes in the inner array. The highly polished outer metal tube (which is earthed for safety, in use) serves to reflect any transmitted UV back into the reactor tubes and to protect the user from intense UV light as well as a failure of a high-power lamp (e.g. Hg-lamp). The photoreactor (including any photoreactor according to the aspects of the invention) preferably has interlocks to protect against overheating, earth, voltage, air and liquid coolant circulation failures. The photoreactor may also have a variable power supply so that the lamp can be run at varying power (e.g. 1.5 to 5 kW) to match the quantum efficiency of any photochemistry and flow rate of pump used.

Figures 3 and 4 show plan views of the upper end and lower end of the photoreactor respectively. The components are generally as illustrated in Figures 1 and 2. Figure 4 shows air outlet 40 where cooling air (and any ozone) flowing over the lamp (in use) by use of a fan is released to atmosphere.

Figure 5 shows a schematic cross section of a photoreactor 2, with a single concentric array of reactor tubes 28. A fan 19 is mounted at one end of the

photoreactor 2 to direct cooling air into the lamp receiving aperture. Cooling air inflow A generated by fan 19 flows over the high powered (1.5-5 kW) medium pressure Hg UV lamp 42 situated in lamp receiving recess 44 and flows out of cooling air outflow B whilst the lamp 42 is operating. Reactant solution in reactor tubes 28 is pumped into reactor inlet 32 using a flow pump (not shown) and flows at a predetermined flow rate (of e.g. 0.01 to 500 ml/min, preferably 2 to 75 ml/min) through the reactor tube 28, through fluid connectors (not shown in Figure 5) in end caps 6, 8 and after passing through all reactor tubes 28 flows out of reactor outlet 34. Cooling liquid is pumped into the cooling/coolant jacket 11 formed by inner tube 20 and outer tube 4 sealed in end caps 6, 8. The coolant jacket 11 allows coolant (e.g. water or water/glycol) to enter through cooling water inlet 10 in the outer tube 4, to circulate and flow around fully surrounding the reactor tubes 28 and exit out of cooling water outlet 12 in outer tube 4.

The footprint of the reactor can be compact for such a powerful device. A photoreactor as illustrated in the Figures but with 36 reactor tubes (each with an inner diameter of 3 mm) was found to operate flawlessly at both high (36 mL/min) and low (5 mL/min) flow rates of reactant without any observed overheating of the reactor or contents. Although an ordinary mains water supply could be used for cooling, more consistent results were obtained using a commercial stand-alone 2.5 kW circulating chiller (water/glycol). The invention is further illustrated, but not limited, by the following examples. Examples 1 and 2 and Comparative Examples A and B.

In these Examples, a comparison was made using maleimide [2+2] flow photochemistry between an FEP reactor and a photoreactor according to the present invention (Parallel Tube Flow Reactor, "PTFR") with a single array of 26 concentric reactor tubes (each with an inner diameter of 4 mm) and no outer tube and using, in both cases, a 400 W water cooled Hg-lamp. The general reaction investigated is as shown in the equation:

x mL/min

The results are shown in Table 1.

Table 1

The initial screening with a [2+2] reaction between N-methyl maleimide and hex-l-yne gave a productivity of 3.41 g/h, which approached the productivity of an optimized 3-layer FEP reactor (3.62g/h). Similarly, maleimide and propargyl alcohol gave a productivity of 2.13 g/h compared to 2.35 g/h for the FEP reactor. These were excellent and perhaps surprising initial results, especially considering that they were obtained using the reactor without an outer tube and inner surface reflector and with just a single concentric array of tubes. Examples 3 to 9.

Examples 3 to 9 were conducted using the photoreactor generally as shown in Figures 1 to 4 and described above but with 36 reactor tubes each of 3 mm internal diameter. After trialling a few reactions that had been previously studied, we found 3 kW to be a convenient power setting to assist in rapid scale-up. A number of reactions were investigated with the chemistry of the reactions as indicated in Table 2 and the results as indicated in Table 3. When set at 3 kW, the photoreactor of the invention gave a productivity approximately 10 times greater than the corresponding

productivity of a three-layer FEP flow reactor with a 400 W lamp. For example, the [2+2] cycloaddition of N-methyl maleimide and trichloroethene gave 2.85 g/hr, 68% at optimised flow conditions (0.1 M, 3 ml/min) for a 400 W lamp equipped FEP reactor. When the lamp in the photoreactor was set to 3 kW, the reaction was at nearly identical conversion when a 0.2 M solution was run at 15 ml/min giving a

productivity of 28.8 g/hr (Example 5). This enabled the isolation of 64 g of product (66% yield) in just 2 h 10 min whereas the 400 W FEP flow reactor gave 62 g in 22 h. This ability to cut down reaction times 10 fold is a great advantage of the photoreactor of the invention.

The [2+2] cycloaddition of malemide and propargyl alcohol (Example 4) is a reaction that can be problematic to scale up on account of formation of insoluble byproducts. A 400 W FEP reactor gave 2.35 g/hr productivity with optimised flow conditions of 0.1 M at 4 ml/min, but for extended runs the insoluble material began to adhere to the walls of the FEP and reduced UV transmission. When using the photoreactor of the invention at 3 kW, we were able to flow 2.5 L of a 0.1 M solution at 36 ml/min and still maintain almost complete conversion with a productivity of 24.2 g in 70 min (64%, 21.2 g/hr).

3,4,5,6-tetrahydrophalic anhydride and cis-2-butene-l,4-diol undergo an efficient [2+2] cycloaddition-lactonisation sequence to a tricyclic lactone upon direct UV irradiation in batch (Example 6). Repeating the batch irradiation at 0.4M with 1% of isopropyl thioxanthone gave a 77% yield of lactone in just 2h of irradiation - a 15- fold increase in productivity over previously published results. Transferring these optimized conditions to the photoreactor (0.4 M solution at 36 mL/min) gave 1,538 g of pure lactone product in just 9.3 h. of irradiation - the product crystallising out in the receiving flask. In a 24 hour run this productivity would enable the synthesis of almost 4 kg of product. The ability to produce kilograms of such a complex product from low cost starting materials highlights the potential of this reactor.

The classic 'Cookson's Dione' product (Example 7) is produced by an intramolecular [2+2] reaction of the adduct formed from a Diels- Alder reaction between cyclopentadiene and benzoquinone which had previously proved to be highly productive in our FEP reactors. This photocycloaddition is so productive, we were unable to demonstrate its maximum productivity as we could not safely make enough of the Diels- Alder adduct in a university fume-hood and also we were at the maximum flow rate of the pump used. For example, at 1.5 kW power a single 140 min run of a 0.5M solution (5 L, 36 mL/min) gave 387 g of pure dione product upon trituration (89%). This equates to 4.0 kg in a 24 h period. In a certified process laboratory, it is predicted that the full 5 kW power lamp rating using the inventive reactor would deliver 13.3 kg of Cookson's dione per 24 h.

We developed a scalable route to a bridged pyrrolidine by a 'crossed' [2+2] cycloaddition (Example 8). After extensive reaction screening in batch, we found thioxanthone sensitizers to be most efficient in enabling a high yielding [2+2] cycloaddition. The reaction proceeded with a productivity of about 17 mmol/hr with just 1% sensitizer in a 400 W batch experiment (0.4 M). Using these optimized conditions, a total of 4.8 moles of starting material in 12 L of MeCN was irradiated in the photoreactor as in Figure 1-4 at 9 ml/min to give 1,082 g (86% yield) of pure crossed [2+2] product in 22.2 hours.

Griesbeck and Oelgemoller (in A. G. Griesbeck, A. Henz, W. Kramer, J. Lex, F. Nerowski, M. Oelgemoller, Helv. Chim. Acta, 1997, 80, 912 - 933) reported the photodecarboxylative cyclisation of a phthalimide-potassium salt to a hydroxy lactam in the presence of acetone as a triplet sensitizer. This reaction was interesting to test on kg scales with a photoreactor according to the invention as the starting material is readily available in large quantities. On trialling this reaction with a 125 W batch reactor we found that the acetone solvent sensitizer was not necessary and acetonitrile was a more convenient alternative co-solvent with water. Irradiation of a 0.2 M MeCN solution in a 150 ml quartz batch reactor gave full conversion in 2 hrs (4.6 g, 81%, 12.2 mmol/hr). By increasing the concentration twenty-fold, purification of the product was made considerably more efficient as on evaporation of the MeCN the bulk of the pure product precipitated and was collected by filtration. By using a quartz inner tube as a light filter in the photoreactor, a total of 6.4 moles of the phthalimide- potassium salt was irradiated in 32 L of MeCN/H20 at 30 ml/min to give 1,032 g of pure lactam (85% yield) in 18 hrs. (Example 9).

Example 6 to 9 have all proved to be excellent low-cost reactions with greater than 1 -kilogram productivities in a 24 h processing period. It is perhaps surprising for such high-powers and concentrations involved that foul-up of the reactor was much less than expected in long runs. In general, we found that as long as the starting material was very pure, and the reactor was clean to start-with, then foul-up proved not to be an issue for the examples studied. Part of this is undoubtedly due to the high flow rates involved. Potential fouling-up during a run can easily be identified by NMR as a drop in conversion. At the end of the run the reactor can be easily cleaned by flushing with 2-3 volumes of polar solvent (MeOH or DMSO/H₂0).

The reaction of Example 5 gives an optimised productivity of 2.85 g/h (68%) in a 400W FEP reactor. When set to 3 kW the inventive photoreactor as in Figures 1 to 4 gives a productivity of 28.8g/hr (66%) for the same concentration. This is 10.1 times the productivity using only 7.5 times the UV power, making the inventive photoreactor almost 30% more efficient on a watt-per-watt basis.

As the UV lamp is not an integral part of the device, it can easily be removed and replaced with a lamp of different wavelength to match other types of

photochemistry e.g. visible light photocatalysis. The photoreactor is also small enough to be situated in a lab fume-hood, yet can meet the productivity demands of a process chemistry environment. The photoreactor may be compact and may be constructed so that it can be housed in a standard fume-hood and be safely operated with a 1.5-5 kW medium pressure Hg-lamp. On scale up higher power lamps may be used up to 60 kW or higher.

 Example Run Lamp Concentration Flow rate Yield Productivity time power (M) (mL/min) (%) (g/h)

(h) (kW)

3 1.17 3 0.1 36 65 25.2

4 1.17 3 0.1 36 64 21.2

5 2.22 3 0.2 15 66 28.8

6 9.26 3 0.4 36 80 166

7 2.31 1.5 0.5 36 89 167

8 22.22 3 0.4 9 86 48.9

9 17.78 3 0.2 30 85 57.9

Table 3

Reference Numerals

2 Photoreactor

4 Outer tube

6 Lower end cap

8 Upper end cap

10 Coolant/cooling water inlet

11 Coolant jacket

12 Coolant/cooling water outlet

14 Fixing screw

18 Lamp receiving aperture

19 Fan

20 Inner tube 22 Lower spacer

23 Lower end piece

24 Upper spacer

25 Upper end piece

26 0 ring seal

28 Inner reactor tube

30 Outer reactor tube

32 Reactor inlet

32a Reactor inlet aperture

34 Reactor outlet

34a Reactor outlet aperture

36 Connector groove O ring seal

38 Connector groove

40 Air outlet

42 Medium pressure Hg UV lamp

44 Lamp receiving recess

A Cooling air in flow

B Cooling air out flow

Claims

1. A photoreactor comprising, a first end piece, a second end piece, a plurality of elongate reactor tubes extending between the first end piece and the second end piece, the reactor tubes being axially distributed so as to define a lamp receiving recess, and a plurality of fluid connectors connecting at least some of the reactor tubes.

2. A photoreactor as claimed in claim 1, wherein each reactor tube is connected by a fluid connector so as to be in fluid connection with at least one other reactor tube, preferably with at least two other reactor tubes.

3. A photoreactor as claimed in any one of the preceding claims, wherein the reactor tubes are connected by fluid connectors so that fluid flowing through a first reactor tube flows in a first direction and in an adjacent reactor tube flows in a second direction, preferably the opposite direction.

4. A photoreactor as claimed in any one of the preceding claims, wherein all the reactor tubes are connected by fluid connectors so all the reactor tubes are in fluid connection.

5. A photoreactor as claimed in any one of the preceding claims, wherein the fluid connectors are comprised in the first end piece and/or in the second end piece.

6. A photoreactor as claimed in any one of the preceding claims, further comprising a coolant jacket surrounding the reactor tubes, the coolant jacket allowing circulating coolant to flow around and over the reactor tubes, in use.

7. A photoreactor as claimed in any one of the preceding claims, further comprising an inner tube situated in the lamp receiving recess.

8. A photoreactor as claimed in any one of the preceding claims, further comprising an outer tube surrounding the reactor tubes.

9. A photoreactor as claimed in claim 8, wherein the inner tube and outer tube are sealingly connected to the first end piece and to the second end piece, thereby forming the coolant jacket containing the reactor tubes.

10. A photoreactor as claimed in claim 9, wherein the outer tube comprises a coolant inlet and a coolant outlet.

11. A photoreactor as claimed in any one of the preceding claims, comprising at least a first reactor inlet connected to a first reactor tube and at least a first reactor outlet connected to a second reactor tube.

12. A photoreactor as claimed in any one of the preceding claims, wherein each reactor tube is substantially straight.

13. A photoreactor as claimed in any one of the preceding claims, wherein the reactor tubes are substantially parallel.

14. A photoreactor as claimed in any one of the preceding claims, wherein the reactor tubes are distributed in at least one concentric array.

15. A photoreactor as claimed in any one of the preceding claims, wherein the reactor tubes are distributed in an inner concentric array and an outer concentric array,

16. A photoreactor as claimed in claim 15, wherein each reactor tube in the outer concentric array is offset with respect to a reactor tube in the inner concentric array.

17. A photoreactor as claimed in any one of the preceding claims, wherein the outer tube comprises a reflective inner surface.

18. A photoreactor as claimed in any one of the preceding claims, wherein the inner tube comprises quartz, Pyrex, Vycor or ceramic.

19. A photoreactor as claimed in any one of the preceding claims, comprising a light filter located between the lamp receiving recess and the reactor tubes.

20. A photoreactor as claimed in any one of the preceding claims, wherein each reactor tube comprises quartz, Pyrex, Vycor, ceramic or polymer.

21. A photoreactor as claimed in any one of the preceding claims, further comprising a lamp located in the lamp receiving recess.

22. A photoreactor as claimed in any one of the preceding claims, further comprising a cooling fan located so as to direct gas flow in the lamp receiving recess, preferably over a lamp when inserted and in use.

23. A photoreactor as claimed in either claim 21 or claim 22, wherein the lamp is an ultraviolet lamp.

24. A photoreactor as claimed in claim 23, wherein the lamp is a medium pressure Hg ultraviolet lamp.

25. A photoreactor as claimed in any one of claims 21 to 24, wherein the lamp has a power of 0.2 to 60 kW.

26. A photochemical process comprising a) providing at least a first reactant solution, and b) flowing the first reactant solution through a photochemical reactor as claimed in any one of the preceding claims whilst irradiating the first reactant solution with light of a predetermined wavelength.

27. A photochemical process as claimed in claim 26, wherein the predetermined wavelength is in the range 200 nm to 800 nm,

28. A photochemical process as claimed in claim 27, wherein the predetermined wavelength is in the range 220 nm to 400 nm, preferably 250 nm to 400 nm.

29. A photochemical process as claimed in claim 27, wherein the predetermined wavelength is in the range 350 nm to 650 nm, preferably 400 nm to 625 nm.