Photocatalytic Reactors and Light Sources

Photocatalytic reactors are used for water splitting, air purification , photochemical reactions and photo chlorination reaction. This reaction often occurs in the presence of photon and catalyst. In this project we would be talking about photocatalytic reactor for air purification. Firstly when dealing with air purification we need to know what the volatile organic compounds (VOC)means for enlightenment purpose.

According to (Melanie et al, 2010) The volatile organic compounds is defined as organic chemicals that when discharged into the atmosphere can react with sunlight and nitrogen oxides (NOx) in other to produce tropospheric (ground-level) ozone.

The removal of VOCs using photocatalytic process as to do with surface reaction process, which involves 2 important steps which are;

         Transfer of VOCs to the reaction surface first.

         Decomposition of VOCs by the photocatalyst.

Thus, it was found that VOC convective mass rate, the reaction surface area and the reaction rate are the most important performance factors of a photocatalytic oxidation (PCO) reactor (Henderson, 2011).

Ideally, a PCO reactor structure should consist of; (Zhang et al 2013)

a)  A high specific surface area per unit volume

b)  A support that has small-through channels, which allows high air velocity and high mass transfer

c)  Direct irradiation of Ultra Violet (UV) radiation source on the reaction surface.

 Unless using sunlight, due to electricity charges and bulb replacement, the most expensive component of any photo catalytic reactor is often the light source. Hence; effective utilization of the produced photons is very important, in other to ensure that the emitted photons comes in contact with photocatalyst and initiate oxidation. In collaboration with what has been said earlier on, for the catalyst in a reactor to remain without irradiation, the surface of the reactor will have to receive irradiation from the light source to avoid flow path through the reactor exist.

Photocatalytic reactors are mainly classified according to their UV lamps location and where photoreaction area takes place in the reactors, this has led to different photocatalytic reaction efficiency, irradiated areas and pollutant mass transfer for each of the various photocatalytic reactors. (Zhao and Yang, 2003; Mo et al., 2009).

Photocatalytic reactors

Photocatalytic oxidation (PCO) has been applied mostly in the commercial sector where PCO functions as a secondary to the main function. Commercial sector application includes glazing, paving stones, wall paper, and paint to mention a few. These products are typically activated solely by sunlight with photocatalytic air purification tending to be given less significance than the “self-cleaning” aspect of these products. Non-the-less this marks a major shift from conventional air purifying systems. (Michael.B et al 2006)

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An air purifying capability can be incorporated into construction materials, surface finishes, even clothes. Devices solely intended for purification purposes are still an important technology required to meet the need for clean air. Immobilized TiO2 is being employed in place of conventional purifying units, or incorporated to form hybrid devices. These are typically not activated by sunlight but by UV lamps, so achieving greater efficiencies, and can be located in areas where natural light is minimal if not non-existent.

TYPES OF PHOTOCATALYTIC REACTOR

Generally, for a photocatalytic reactor of an air purifier to perform efficiently , it depends on the design. In the reactor, air contaminant is removed by surface reaction of 3 important steps, which includes;

• The pollutants been transferred to the catalyst surface
• Adsorption/desorption takes place
•  And the photocatalyst decomposes the pollutants.

Hence, a photocatalyst reactor main performance parameters are ; The transfer of mass rate, reaction at surface area and kinetic area (Mo et al 2009). Usually, a photocatalytic reactor should comprise of two parts, which are; UV light source and the structure of the reactor. This structure support both the photocatalyst and the airflow channels. (Zhang et al. 2013)

We would be discussing about different types of photocatalytic reactor with their various unique operation modes and functions. In terms of categorizing, reactors are found to be functions for different purposes, but in this project we would be talking about Photocatalytic reactor for air purification- Flat plate reactors, Honey comb monolith reactor, Fluidised bed reactor, Glassfibre/membrane reactor, Annular reactor and Optical fibre reactors

 Flat plate reactors

 Flat plate reactor is made up of two flat glass plates with a certain gap between them, in which cleaned fluid is passed through. The interior surface of each plate consist of a coated catalyst, that has an external light source emitting the catalyst. The catalyst layer is quite tiny in thickness which makes it easy for the entire catalyst surface to be illuminated by the light. The light sources is found to have behind them be a reflector, this reflector plays a role of directing the available radiation onto the catalyst. This type of reactor is found to be the most elementary reactor and basically, utilizes available light poorly. (Michael. B et al. 2006)

Fig1: Flat plate reactor, Brandi et al. 1991

Honeycomb monolith reactors

A honeycomb monolith reactor is made up of a certain amount of channels, in which each individual channel with typical internal dimensions of around 1mm; the cross-sectional channels has either a square or circular shape and catalyst in this reactor can be found coated with very thin wash onto the walls of the channels, (Hayes et al., 1992). Honeycomb monolith reactors gives negligible pressure drop and it’s configurations are commonly found in automobile exhaust systems used for emission control and also to reduce NOx in power-plant flue gases. A very thin layer of catalyst is coated onto the walls of the channels. The main advantages of this type of photocatalytic reactor is the fact that;

1. It has a low-pressure-drop
2. It has high to surface area to volume ratios (Jaun et al, 2003).

 This reactor was used to carry out a research, According to Raupp, who analyzed reactors using different monolith formations like; square channeled monoliths that consists of various dimensions and also porous cylindrical ceramic monoliths. To attain energy-efficient of a photocatalytic reactor designs applicable to monolith reactor, mathematical modeling such as air flow , photon flux field and mass transfer in photocatalytic monolith reactors had to be carried out.(Hossain et al, 1999; Raupp et al., 2001; Votruba et al., 1975). Suzuki et al. (1991) reported how photocatalytic monolith is used for the oxidative destruction of odours , while Sauer and Ollis (1994) carried out a research on the photocatalytic oxidation of acetone in the air with the use of near-UV illuminated TiO2 coated located on the surface of a ceramic honeycomb monolith. An example of the monolith reactor is the one used by Raupp et al. (2001), showed in Figure 2

Figure 2: Monolith reactor used by Raupp et al., 2001

Fludised bed reactors ( FROM HERE UNEDITED)

The way fluidized bed reactor is designed allows them treat fairly high gas feed rate ,that is gas can pass directly throught he catalyst bed. In a fluidized bed reactor, the catalyst and reactant have a good contact between them, this is due to the design of the reactor and high surface area of the catalyst. Nonetheless, there is a low pressure drop , low mass transfer resistance and the catalyst limit the UV lights penetration (Michael et al. 2006) all these are advantages this type of photocatalytic reactor. For fluidized-bed reactors, the air streams moves in a vertical way directly to a transparent container that comprises of catalyst bed filled with the catalyst bed as shown in the diagram below. A catalyst is made up of Tio2 silica gel infused in the catalyst using a sol gel method, As a result of this ,the particles displays a smooth/bubbling fluidization, silica gel particles has the diameter of 0.25-0.45mm. (Michael et al, 2006). Zhang et al (2006). when a research was carried out to check how photocatalyst degrade mixed gaseous carbonyl compounds, photocatalyst was found to have a high adsorption performance rate and with an excellent photocatalystic activity for four carbonyl compound mixtures.

 Further studies were carried out for fluidized bed reactors by various researcher such as;

• Cant and cole (1992) studied about ammonia oxidation in a fluidized bed photocatalytic reactor.

• Dibble and Raupp (1992) is described in the diagram below how TiO2 was supported on silica gel; in this research there was a reduction in the momentum of catalyst particles and prevention of catalyst particles flowing with gas , this was achieved due to the glass frit in the overhead effluent tube and also the upper part of the reactor cross sectional area which is larger than the bottom portion.
• Lim et al. (2000) features combination of a fluidised bed and an annular reactor to produce improved fluidized bed reactor, where catalyst formed an annular bed with light source in the middle of the reactor. This annulus fluidized bed reactor was reported to have served as a useful tool for trichloroethylene (TCE) degradation with photon energy utilized efficiently. A quartz filter served as a distributor by providing a uniform fluidization of the catalyst, and a mirror box in square shape that surrounds the reactor to reduce light irradiation and advance the use of deflection and reflection light. Fluidsied bed reactors have more advantage to the fixed plate photocatalytic reactors, due to their effective contact of catalyst-light and reactants(Lim et al., 2000).
• Nam et al.(2002) worked on getting a uniform air distribution in the catalyst bed of the reactor, by placing a light source at the center of the catalyst bed and installed some inlet nozzles at the end of the reactor that aids the distribution

Figure 3: Schematic diagram of the fluidized-bed reactor used by Dibble and Raupp, 1992

Glass Fibre/Membrane Reactors

Glass fibre/ membrane reactor consists of a UV light emitting lamp that has number of layers of matrix material that’s coated in Ti02 surrounding it. This reactor was designed by Pichat et al. (2000) since glass fibre is porous to UV light, effective catalyst/ photon can be produced, if the Ti02 layer is thin. Further studies has been carried out by other researcher working on this reactor type and their basic concerns is with the water purification. They include;

• (Hidaka et al. 2002) formed 2 concentric layers around a centralized light source, using a reactor has comprises of fibre glass cloth.
• Molinari et al. (2001) applied a polymer membrane reactor by studying membranes of various polymer pore sizes, distribution ,materials and thickness.
• Ohitani et al.(2003) combined fiberglass cloth and stainless steel mesh , due to their comparability in photoactivity, the TiO2 immobilized onto fibre glass cloth was found to be more stable.

Figure 5. Glass Fibre, Pichat et al.(2000)

Optical fibre reactor

Optical fibres reactor was designed to remote light transmission and for solid support of the photocatalyst. It was designed by wang et al (2001). It consist of a bundle of optical fibers , which are used as a media of delivering UV within the photocatalytic reactor instead of the normal use of a single UV radiation lamp. (Gracia et al 2012) This makes the reactor different compared to other photocatalytic reactors ,due to their light delivery mechanism. It helps to reduce losses from adsorption and dispersion that has to do with the use of external light source, there by making use of photon economical and its geometry and configuration produces a high surface area. (Michael. et al 2006). The most expensive component of a photocatalytic oxidation system is considered to be the process of a UV radiation source.(Gracia et al 2012)

Figure 4: Schematic diagram of the optical fiber photocatalytic reactor used by Wang et al., 2003

Annular Reactors

Annular reactor is designed with two concentric cylinders that creates an annular region with a particular gap. The light source of this catalyst was found to be located at the center of the reactor and catalyst was found on the interior wall of the outer cylinder with the catalyst film coated having a thin thickness on the surface of the reactor in other to permit catalyst irradiate by the UV source. Also light source can as well be located outside of the reactor that the catalyst is coated on the two concentric cylinders (Gracia et al 2012). Basically the cross sectional area of the annular reactor is usually small, this is enables the reactor high gas flow velocity in other for products desorbing from the surface is quickly removed (Larson et al., 1995).

Figure 4: Schematic diagram of the annular reactor used by Larson et al., 1995

2.4.6 Light sources

The radiation source, ultraviolet radiation and specifically near-ultraviolet radiation, is

a very important component of the photocatalytic process. The light source plays a

critical role (as the energy provider) on the photocatalytic degradation of the

pollutants: the photocatalyst activity depends strongly on the light-irradiation (energy

per unit area) or the photon flux on the surface of the catalyst.

Ultraviolet radiation refers to electromagnetic radiation in the 10-400 nm wavelength

range. Radiation in the 10 to 200 nm is considered as Vacuum UV since it is absorbed

by air, UVA covers from 315 to 400 nm, UVB from 280 to 315 nm and UVC from

200 to 280 nm.

The band gap of TiO2 anatase is 3.2 eV and the irradiation portion that can participate

in the photocatalytic reaction is the one below 388 nm; commonly near-UV radiation

with the wavelength of near 300-370 nm. This type of lamp is used to provide the

energy to induce the process of the photo-sensation. While on the other hand the biohazardous

UV-254 nm is avoided to be employed.

Artificial UV lamps can power photocatalytic processes and are made of different

metals including mercury, sodium, zinc/cadmium and rare gases (neon, argon). The

mercury emission lines are usually in the desired range of energy for driving the

photochemical reactions. Artificial UV lamps (Table 2) can be grouped in three

categories (Bolton et al., 1995): low pressure mercury lamp, medium pressure

mercury lamp and high pressure mercury lamp categories.

The heterogeneous photocatalysis can also be driven by solar light since the TiO2

activation spectrum overlaps with the solar spectrum (Nimlos et al., 1993).

Approximately 4%-5% of the sunlight reaching the surface of the Earth is in the 300-

400 nm near-ultraviolet range and this portion of the solar spectrum can be used to

drive photocatalytic reactions (Bolton et al., 1995; Matthews, 1993; Wilkins and

Blake, 1994).

Some disadvantages of solar energy, however, are its intermittency and variability

with both factors being geographically dependant (Wilkins and Blake, 1994). Bolton

et al., (1995) have mentioned that solar energy cannot be used effectively for

homogeneous photochemical processes since typical reagents such as H2O2 and O3 do

not significantly absorb radiation above 300 nm and none of the radiation received at

the surface of the earth is below 300 nm.

Therefore, the application of solar light is clearly favored in photocatalytic

heterogeneous processes versus its application in homogenous photocatalytic reactors.

GENERAL ADVANTAGES AND DISADVANTAGES OF PHOTOCATALYTIC REACTORS FOR AIR PURIFICATION (H. Ren et al 2017)

1. For fixed plate reactor

Advantages

• It has simple geometry
• It has low pressure drop

Disadvantages

• This type of reactor has Low convective mass transfer rate and small reaction area(Mo et al. 2009)

2. Honeycomb monolith reactor

Advantages

• The mechanical strength of a monolith reactor is high.
• It has a high reaction area
• There’s an Intermediate convective mass transfer rate.

Disadvantages

• When monolith is too thick, there is a low photon utilization rate , which leads to reduction in radiation exposed to the perforated walls.

3. Annular reactor

ADVANTAGES

• It has simple geometry
• Low in pressure drop

DISAVANTAGES

• There’s a low convective mass transfer rate within the reactor
• It’s has Small reaction area (Zhao et al. 2003) (Mo et al 2009)

4. Fluidized-bed reactor
    

ADVANTAGES

• Rate of gas-feed is high
• Pressure drop is low
•  The reactor has high convective mass transfer rate (Zhao et al 2003)

DISADVANTAGES

•  Photocatalyst isn’t removed easily and there’s an entrainment (dribble et al 1992)

 Commercial applications

Of the reactor types described in section 4.2 the two most commonly found in commercial

applications are the flat plate and fibre/membrane configurations. These are

occasionally combined with other air purifying technologies, typically HEPA

filtration and/or air ionisers, to form a final product. Table 2 lists a selection of

commercially available products that are being marketed for domestic and light

industrial use.

There is an ever-increasing volume of PCO products becoming available from a

wide range of suppliers, mostly based in Japan and the USA. Not only are these

product being seen as a benefit to the indoor environment of homes, offices, and

industrial complexes, but PCO products are being used to prolong the life of agricultural

produce, as with the Nippon Muki Co.’s Freshlong® [42] that eliminates

the decay-promoting gas ethylene, and even as a potential resource against bioterrorism,

as with the KES AiroCide [43], which has been proven to neutralize

anthrax spores.

Conclusion

The ability to coat almost any surface with photoactive material is one of the fundamental

reasons why photocatalysis has been received with such enthusiasm. An

extraordinary range of applications have already been commercialised and more still

are in the research phase.

At the University of Nottingham researchers are working to incorporate the photocatalysis

process in natural and mechanical HVAC systems as well as natural lighting

schemes [44]. Fundamental research is being conducted in collaboration between

the School of the Built Environment and departments of Chemistry and Materials

engineering. Commercial products are being designed and tested in conjunction with

industrial partners such as Pilkington and Baxi.

The future prospects of PCO look promising. The photocatalytic ability to

eliminate pollutants, rather than change their phase, using non-hazardous and environmentally

safe materials, while not requiring an electrical supply if using natural

light, is a major advantage over competitive technologies. Of great significance is

research currently underway to find a photocatalyst that can be activated by visible

as well as UV light. Should this be achieved commercial activity in this area would

no doubt increase significantly.

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