Question

(a) What type of reactor is ideal for treatment with an AOP? (b) Why?

(c) Practically, how would you design the reactor to achieve your desired goals in parts (a) and (b)?

Answer 1

a) A photocatalytic reactor based on UV light sources and a catalyst is used for purifying and disinfecting wastewater using Advanced Oxidation Processes (AOP). In ths, the catalyst is supported over glass sheets as fixed strips within the reactor at an established distance from the UV light source. The wastewater is treated as "batches" being recirculated and returned to the circuit. The wastewater is fed through the lower end of the reactor and submitted to a strong turbulence when passing through perforations or holes made on the supports forming the floor of the reaction chambers.

b) Photocatalysis consists on the modification of the reaction rate of a process in the presence of light and of a so-called photocatalyst. The photocatalytic reactors operates using catalyst suspended in the solution or immobilized on various supports. Titanium dioxide (TiO2) is a promising photocatalyst, when exposed to sunlight or UV rays. TiO2 is relatively cheap, non-toxic, insoluble in water and very resistant to most chemicals. It shows the highest photocatalytic activity and resistance to so-called anodic photo corrosion. Additionally, the photocatalytic processes on titanium dioxide can be also initiated by solar radiation.

c) A great research effort has been done in the last years in order to develop more efficient photocatalytic systems and some applications are already available at the commercial scale. However, the design of a reactor where these processes take place is still challenging, since not only the phase contact, turbulence or mixing are important issues but the interaction of light, either solar or artificial, adds a new aspect to integrate in the equipment.

The reactor was designed on the basis of fluidized bed concept, with three phase contact pattern and also
maximizing the UV irradiation area. A fundamental issue regarding the successful implementation of photocatalytic reactors is the transmission of irradiation in a highly scattering and absorbing medium composed of water and fine
TiO2 particles. When the reactor consists of three phases that is liquid, solid and gas, it is called as three phase reactor. The feed is in liquid form, catalyst is in solid form and air is in the gaseous form, enters the system by aeration done to support the photocatalysis.

The reactor is designed in form of sandwiched chambers to utilize UV light efficiently. The reactor has staged contact pattern, the inlet chamber is the first stage where catalyst makes contact with feed. The second stage is the contact made in the two compartments located at the ends. The chambered construction was thought in order to increase the maximum residence time of the fluid. The two stages allow the feed to be treated more efficiently with
through mixing. The reactor consists of five vertical chambers and one common lateral chamber at the bottom which connects the five chambers. From the five vertical chambers, two adjacent compartments from the central compartment are made water proof and are sealed from all sides except from the top. These two compartments house the UV lamps. The reactor in the bottom compartment consists of aeration system, which is made of copper tube. The catalyst commonly used is Titanium dioxide. The catalysts are coated on the solid silica gel particles using sodium acetate (LR grade). The sodium acetate is used as a suitable binding agent, as it is cheap, ease of coating and also it is one of the least toxic materials used for coating of the Titanium dioxide.

It consists of feed tank and pump to transport the feed to reactor. The four outlets from the reactor are put back into the feed tank. The aeration system is connected to an air header made of copper which distributes one compressed air input to four outputs. The aeration tubes are placed into lower compartment. Artificial light sources are also needed:

• to reflect or focus the emitted light to a reactor.
• to wrap the light source with the reactor.

The main artificial sources are mercury lamps.