Dye effluents with low BOD/COD ratio and varied chemical structures are usually very recalcitrant to microbial degradation. Therefore, different process was used for the treatment of dye effluent. Heterogeneous photocatalyst process has been widely used as leading green technology for dye removal. The process utilizes a semiconductor photocatalyst (such as TiO2 or ZnO) and UV light to oxidize the recalcitrant organic compounds to inorganic ions, carbon dioxide and water. Photocatalytic process needs to photorectors and hydraulic parameters play an important role in mass transfer phenomenon in photocatalytic reactors. These fundamental parameters are flow rate, relative roughness and Reynolds number. This research experimentally evaluates flow rate and artificial relative roughness in order to determine the factors influencing the removal efficiency and reaction rate. For this purpose, a cascade photocatalytic reactor is constructed which consists of similar Plexiglas plates coated by various roughness. Numerical simulation usually overcomes complex reactor models which takes reasonable cost respect to experimental study. Here, OpenFOAM software is also utilized to perform a numerical study. Regime and velocity of sewage are simulated in photocatalytic flow with/without considering relative roughness.
The use of different synthetic dyes in textile industries has increased in recent decay, resulting in the release of dye-containing industrial effluents into natural aquatic ecosystem [
This photoreactor has a number of advantages that include eliminating the need for catalyst separation units as the catalyst is immobilized, creating the flow mixing by non-mechanical method, increasing the transport of oxygen from the gas phase to the photocatalyst surface by providing the flow cascade pattern. The photoreactor was used in order to remove Reactive Yellow 81 (RY81) dye from textile industry effluent, by means of UV-ZnO process. RY81 is a reactive dye composed of 10 Benzene rings and two ?N=N azo bonds. The effect of different artificial roughness and recirculation flow rate in removal efficiency was investigated.
In order to remove the RY81 dye, a photocatalytic cascade disc reactor is constructed, which consists of four 34 cm diameter circular disc made of Plexiglas and immobilized by ZnO nanoparticles (
In this pilot, sewage flow is pumped to the highest level disc, and then it is transferred as cascade flow to lower level discs by small holes provided on each disc (12 small holes with 4 mm diameter). Also, this turbulence flow is spontaneously aerated. Therefore, mass transfer limitation in the reactors with immobilized bed is reduced significantly. A UVC 20 W lamp is located at the center of the reactor, and disc beds are coated by ZnO nanoparticles with area density of 20 gr/m2 [
placed at disc radiuses of 7 and 12 cm. Again, sewage flow in storage supply is pumped to the highest level disc. Total volume of sewage treated in this treatment system is 5 liter which is restored with the flow rates of 40, 60 and 80 cc/sec.
In this study, ZnO nanoparticles with purity of 99% (US research Nanomaterials Inc) are used. Sizes of particles vary from 10 to 90 nm and their surface area and density are 20 - 60 m2/gr and 5.606 gr/cm3, respectively (
Other materials including sodium hydroxide and sulfuric acid, made by The Merck Inc, are used for pH adjustment. Spectrophotometer (Hach Dr400) is utilized for determination of absorption ratio and dye concentration. Also, Fungilab Ultrasonic water bath and a digital weighting scale (PLS360-3-Kern) are employed for dispersing the algometry nanoparticles and material distribution, respectively. In this experiment, the pump is a submerged Soboti WP 3880 and the lamp is selected as a 20 W Philips (UVC).
A two dimensional model is implemented by OpenFOAM software which is a finite volume based open-source computational fluid dynamic (CFD) package.
Initial dye concentration is one of important parameter in photocatalytic process.
creases from 5 to 100, the total time for complete degradation of RY81 increased from 6 to 57 hr. This affects the catalytic activity of ZnO nanoparticles and the number of active site on disk surface. Another impact of increasing dye concentration is related to changing of path length of photons entering the dye solution.
This research also evaluates the effects of flow rate on the RY81 removal. These experiments use dye concentrations of 10, 50 and 100 mg/L; and flow rates of 40, 60 and 80 cc/sec; pH of 7; and a 20W UV lamp. In these experiments, Reynolds numbers are adjusted to be different.
By coating roughness with height of 0.5 cm on the disc surface, total removal run-time is changed.
Time Reduction (%) | Time for Destruction of RY81 with Roughness | Time for Destruction of RY81 without Roughness | Q (cc/s) | C (mg/L) |
---|---|---|---|---|
4.8 | 10 | 10.5 | 40 | 10 |
5.3 | 9 | 9.5 | 60 | 10 |
6.1 | 7.7 | 8.2 | 80 | 10 |
4.5 | 21 | 22 | 40 | 50 |
7.0 | 18.6 | 20 | 60 | 50 |
5.3 | 18 | 19 | 80 | 50 |
5.3 | 54 | 57 | 40 | 100 |
5.8 | 49 | 52 | 60 | 100 |
6.0 | 47 | 50 | 80 | 100 |
Therefore, total dye removal time is decreased about 5.6%. Artificial roughness increases flow velocity of the spaces between baffles so that the velocity and Reynolds
number become two times greater.
Moreover,
Efficiency improvement of a reactor by non-mechanical equipment and roughness change due to using baffles is an economical way for sewage treatment and improvement of photocatalytic reaction rate. Mass transfer rate can also be increased by increasing flow rate leading to turbulence flow. A reactor with roughness property is more reliable and requires low maintenance cost.
Effects of relative artificial roughness are assessed in a cascade disc reactor by CFD simulation. The results demonstrate that the imposed in roughness, separate the boundary layer at downstream of each barrier and vortex is formed between barrier increasing vertical mixture of the flow. Furthermore, experimental results show that mass transfer phenomenon and photocatalytic rate process can be increased by changing hydraulic parameters such as flow rate and roughness. Because dead space is reduced by increasing the roughness, and Reynolds number and flow velocity becomes two times greater without spending energy. Hydraulic condition modifications increase the photocatalytic reaction rate to 26%.
Amiri, H., Ayati, B. and Ganjidoust, H. (2016) Textile Dye Removal Using Photocatalytic Cascade Disk Reactor Coated by ZnO Nanoparticles. Jour- nal of Materials Science and Chemical Engineering, 4, 29-38. http://dx.doi.org/10.4236/msce.2016.412004