the oxidative degradation of fluorescein induced by visible light in the presence of ZnO-APTMS-Au micro/nano-particles in aqueous suspension

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Instituto Tecnológico y de Estudios Superiores de Monterrey

Campus Monterrey

School of Engineering and Sciences

Lab-scale modular platform to study coiled-flow inverters (CFI) as candidates for continuous-flow photoreactor units: A case study based on

the oxidative degradation of fluorescein induced by visible light in the presence of ZnO-APTMS-Au micro/nano-particles in aqueous suspension

A thesis presented by

Chinmay Pramodkumar Tiwari

Submitted to the

School of Engineering and Sciences

in partial fulfillment of the requirements for the degree of Master of Science

In

Nanotechnology

Monterrey Nuevo León, December 4^th, 2020

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Dedication

To my parents and my brother, who have always believed in me irrespective of the situation. Mummy, Papa you always have taught us about this noble Sanskrit verse

from Bhagavad Gita -

“कर्मण्येवाधिकारस्ते र्ा फलेषु कदाचन । र्ा कर्मफलहेतुर्ुमर्ाम ते संगोऽस्त्वकर्मधि ॥ “

and told us to believe in performing duty sincerely without expectations of the outcomes, that has strongly motivated me to get going.

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Acknowledgements

I would heartily thank my advisor Dr. Alan Aguirre Soto, for his constant guidance and support over the time while working. He cheered me when everything went well and continuously motivated me to get going whenever I felt low. He was always there as a mentor and a friend with whom I had a wide range of discussions whenever need be. His sociable nature always welcomed a wide range of intellectual discussions without any hesitations, which I have always cherished and look forward to having more in the coming future. His role in my development has been a special one, and I am truly indebted to him forever for being such a great mentor and look forward to learning under his guidance in the future also.

I would especially like to thank Prof. K.D.P Nigam, who played his role as a mentor monitoring my progress over time and give suggestions whenever need be.

I am also grateful to meet MSc Fernando Delgado Licona, Dr. Enrique A. López Guajardo, Dr. Sara Nunez Correa, and Dr. Alejandro Montesinos for allowing collaborating on projects and allowing to work with them. Working with such excellent researchers and friends had an indelible impact on me, and the learnings from experience will surely guide me for my future professional growth and becoming a better person.

I am also grateful to all the faculty members who taught me over time. Their support and advice were useful and appreciated. I could not imagine having better advisors, better committee members, teachers, and I am truly privileged to have learn under them. I want to dedicate the following Sanskrit verse to my advisors, mentors, and all faculty members seeking their blessings.

गुरुर्ब्मह्मा ग्रुरुधवमष्ुुः गुरुदेवोर्हेश्वरुः ।गुरुुःसाक्षात् परंर्ब्ह्म तस्मैश्री गुरवेनर्ुः॥

I would like to especially thank Dr. Gaurav Chauhan, Dr. Apurv Chaitanya, Gargi, Didi, Dr. Jogender Singh, and Dr. Hafiz Iqbal for their help over time and always making me feel like at home.

All my friends were the backbone throughout my stay away from my family, and I could not imagine myself without them.

(Aida, Fernando, and his family, Daniel, Kendra, Cynthia, Luis, Valeria, Martin, Pedro, Osamu, Maria, Niloufar, Zeinab, all my friends from India and many others)

Without the financial support of Tecnologico de Monterrey and CONACyT, this work would have never been possible.

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Lab-scale modular platform to study coiled-flow inverters (CFI) as candidates for continuous-flow photoreactor units: A case study based on the oxidative degradation of fluorescein induced by visible

light in the presence of ZnO-APTMS-Au micro/nano-particles in aqueous suspension

by

Chinmay Pramodkumar Tiwari

Abstract

Visible light-driven continuous-flow photochemistry has gained widespread recognition lately and is employed in many innovatively designed photoreactors. Out of the two main categories, slurry reactors are found to have a better reputation in terms of achieving competitive photon efficiencies when compared to immobilized catalyst type reactor designs. However, several obstacles had stalled the broad-scale implementation of this beneficial process. A few of the main imminent challenges include combating light attenuation by better mixing in continuous-flow of the suspension to allow the use of the higher photocatalyst content and require lower photon consumption. Also, the difficulties in the fabrication of intricate glass-based photoreactor designs are one of the significant challenges. An inherently better-designed reactor which deals with the common problems of conventional photoreactors is required. This thesis presents a flexible platform to study photoreactors, where a coiled flow inverter—a well-established static mixer design— is used as a micro/milli-fluidic device. The CFI is incorporated as a photoreactor for the first time for a continuous flow photodegradation study of an organic model pollutant, fluorescein, with ZnO catalyst functionalized with APTMS and Au nanoparticles to make it visible-light absorptive. Flow inversions leading to chaotic advection occurring in the CFI combats light attenuation. Due to superlative mixing coupled with a highly efficient visible light source, our photo-CFI stands to be in top slurry reactor designs as per the recently established PSTY benchmark, valued at 2.97×10⁻² (m³ treated water day^-1 m^-

3 reactor kW^-1). A brief study on the uni- and multi-axial light arrangement for complex geometries was used to analyze the effect of geometry/lighting arrangement and ensure uniform irradiation of the photo-CFI. A discussion of dye-degradation products surface interaction with photocatalyst was carried out to analyze possible explanations for an observed destabilization of the suspension during reaction, leading to depositions in the reactor. SLA based additive manufacturing is tested and projected to be a superior alternative for rapid prototyping of intricate transparent photoreactor designs in lieu of conventional glass blowing techniques of complex geometries such as those required for static mixers like the photo-CFI.

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List of Figures

Figure 1. 1 Typical bandgap energies of some MOS presented and relationship between their band gap and wavelength (figure adapted from Riente et al.⁷²) ... 25 Figure 1. 2 Relation of Distance into the medium and Mixing with % Transmittance ... 29 Figure 1. 3 Plot of PSTY vs STY for various designs, highlighting the top three designs in Slurry and Immobilized reactor category ... 36 Figure 1. 4 Schematic of photocatalytic membrane reactor system with visualization of crossflow ceramic membrane (Image adapted from Benotti et al.³⁸,Llorca et al.^,64) ... 37 Figure 1. 5 Schematic of continuous magnetic stirred tank reactor (Image adapted from Vela et al.³⁹) ... 39 Figure 1. 6 CFI in the categorization of Micromixers (Image adapted from Vural Gürsel et al. ⁶⁵) along with the visualization of flow inversion due to bending in a coil (Image adapted from Vashisth,S et al.⁶⁶) 40 Figure 1. 7 Schematic image along with the picture of the photocatalytic reactor and also a visualization of flow and light propagation through the reactor (Image adapted from Claes et al.⁴⁹) ... 43 Figure 1. 8 (a) Schematic of experimental apparatus along with the lamp arrangements used for the study (Image adapted from Yatmaz et al. ⁵⁰) (b) Comparison of bubbles sizes formed in gas-liquid photochemical SDR system with the RPM pointing towards the effective mass transfer (Image adapted from Chaudhuri et al. ⁷¹) ... 45 Figure 1. 9 Experimental set up for PBR (Image adapted from Vaiano et al.⁵¹) ... 48

Figure 2. 1 Lighting configurations for photoreactors. a) Batch photoreactor, b) Continuous-flow

photoreactor, c) Micro-photoreactor, and d) Coiled-flow inverter presented herein. ... 63 Figure 2. 2 Modular platform for the study of coiled-flow inverters (CFI’s) as photoreactors, continuous- flow set-up, platform dimensions and model CFI specifications. ... 67 Figure 2. 3 Fluorescence imaging for the visualization of spatial gradients in irradiance on the outer surface of the photo-CFI. Image analysis shown in red scale for each photograph. ... 72 Figure 2. 4 a) Wide-angle visible-light LED source; b) Measurements of visible-light intensity at different locations inside the reaction chamber for uniaxial top-down irradiation. ... 75 Figure 2. 5 Degradation of fluorescein induced by visible light in the presence of ZnO-APTMS-Au. a) Decay of fluorescence emission as a result of fluorescein degradation. b) Normalized decay in the concentration of the fluorescein model contaminant as a function of time. c) Proposed mechanism for the photoinduced production of reactive oxygen species (ROS) from ZnO-APTMS-Au nanoparticle. ... 77

Figure 3. 1 Proposed mechanism (equations below) for photocatalytic degradation of fluorescein induced by visible-light in presence of ZnO-APTMS-Au particles where charge separation drives the formation of reactive oxygen species, highlighting the proposed role of the binding of the model contaminant to the photocatalyst surface and associated destabilization of the suspension. ... 90 Figure 3. 2 a) Representation of fluid mixing and flow inversion b) Observed sedimentation initially versus towards the end of photodegradation reaction c) Representative figure (inspired by Kurt et al.¹⁵) for single- particle (in red star shape) tracking with high-speed camera (200 fps) for analyzing the radial movement in helical coil tube with respect to time. The inner wall region (marked ‘-‘) and the outer wall region (marked ‘+’) of the tube are separated by the dotted line. Fc indicates the direction of the centrifugal force that is perpendicular to the flow direction. ... 93 Figure 3. 3 FTIR spectra comparison for ZnO (commercial), ZnO_APTMS_Au (Before degradation) and ZnO_APTMS_Au (After degradation) indicating variation in peaks ... 97 Figure 3. 4 a. Incongruity in linear fit for pseudo first order ln(F/F0) vs. t curve for degradation curve and control experiments (F=Concentration in mol/lit of fluorescein at time t, F0=Initial fluorescein concentration in mol/lit) b. Pseudo second order fit 1/F (lit/mol) vs t (time) for degradation curve and control experiments c. Pseudo second order linear plots [1/F (lit/mol) vs t (time)] with varying kinetics at different time intervals for dye degradation using ZnO-APTMS-Au as photocatalyst d. UV-VIS spectra for fluorescein degradation with respect to time [consisting of both Fl (~521nm) and FL anion dimer peak (~551 nm)] ... 99

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Figure 3. 5 Previously proposed mechanism for the complete degradation of Xanthene dyes, e.g.

fluorescein from observations by Yu et al.¹⁷,Ou et al.¹⁸, and He et al.²⁴ ... 104

Figure 4. 1 Visualization of the superiority of SLA over FDM ... 116

Figure 4. 2 Explanation of XY-plane resolution and Z- resolution for Formlabs Form 2 ... 120

Figure 4. 3 Effect of surface roughness on light scattering ... 123

Figure 4. 4 a) Qualitative and b) Quantitative difference between Unprocessed and Post-processed printed parts (preliminary experiments) ... 126

Figure 4. 5 Relationship between optical scattering (TIS) and surface roughness (RMS) (at 𝜃𝑖 =60, 𝜆 = 450 𝑛𝑚, R0 =0.2, T=0.8, A=0)... 128

Figure S 1 FTIR spectrum of the ZnO functionalized with APTMS. ... 139

Figure S 2 UV-vis spectra of aqueous ZnO-APTMS-Au/Fluorescein solutions with fluorescein concentrations of 10 μM and 1 μM. ... 140

Figure S 3 Blueprints of: a) Front and back panels; b) Side panels; c) Top panel (light source inlet) and d) Base panel (for the reaction chamber and monitoring chamber. ... 141

Figure S 4 Different views of the modular photocatalytic platform with CFI. ... 141

Figure S 5 Sedimentation and deposition of the catalyst at flow rates below 80 mL/min. ... 142

Figure S 6 Orange tint on CFI wall due to the formed by-product and low degradation. ... 142

Figure S 7 Algorithm for the design and operation of the continuous photocatalytic platform ... 143

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List of Tables

Table 1. 1 Bond dissociation energies for organic molecule bonds (adapted from Blanksby et al.⁸) ... 15 Table 1. 2 The comparison between various designs of photocatalytic reactors with respect to their PSTY values ... 32

Table 3. 1 Rate constant values for various phases with respect to time intervals ... 101 Table 4. 1 Comparison of various techniques for transparent fluidic devices fabrication ... 114

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Chapter 1 1. Introduction

1.1 Context and Problem Statement

From the genesis of the industrial revolution, as technology advanced and people started using fossil fuels, air/water pollution have slowly become one of the grave issues faced by humanity today. As of present, there is no sign of the proper damage control. Air pollution-related premature deaths account for 7 million people, and water pollution- related deaths, explicitly diarrheal deaths, account for close to 1 million as per reports from the World Health Organization. Many diseases are caused by water and air pollution, namely cholera, diarrhea, typhoid, hyperreactive airway diseases like rhinosinusitis, allergic rhinitis, pharyngitis, and sometimes leading to asthma, are becoming very common.¹ Apart from developing new technologies to make inherently safer processes which are less/non-polluting, it is also necessary to be less reliant on the conventional/

non-renewable sources of energy. Strong emphasize should be laid on the usage of sustainable renewable energy sources to curb down the generation of pollutants by developing techniques following principles of green chemistry.

One such perennial energy source is the Sun, and the functioning of our planet and the living entities in it directly or indirectly depend on the effect of sunlight, which has played a significant role in day-to-day processes and the evolution of life forms. During the brink of the industrial revolution, one of the other exciting fields based on the sunlight and chemical interactions, i.e., photochemistry, was found and paving its way.

The contribution of photochemistry to basic research since its inception is undeniable— from peering into the nature of molecular orbitals to elucidating the

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interactions between photons and matter. Nevertheless, it took several decades for some photochemical processes to permeate into industrial applications, such as the light- induced polymerization of coatings,² sealants and adhesives,³ the lithographic reproduction of digital patterns for electronics,⁴ and the synthesis of intermediate to final chemical products⁵ (solvents, polymers, specialties, and pharmaceuticals). While the advantages provided by light have been exploited in these applications— namely spatiotemporal control of chemical reactions, access to free radical chemistry, and access to otherwise unavailable isomers—, their development and adoption have generally been halted by the challenge of engineering appropriate reaction systems that can compete with thermal-activated chemical processes in terms of efficiency and productivity.⁶ This advantage explains why most of these examples rely on free radical chemistry, leveraging its chain reaction nature to counteract photon generation and transfer's compounded efficiency drawbacks. Additionally, some of the final products' relatively high cost may balance out the added expense that comes from the photonic activation.

Today, as efficiency has become a central topic in virtually every aspect of engineering and science, an ever-expanding plethora of high-efficiency light sources, chemical processing operations, and materials are being developed. This has lowered the hurdle for the adoption of light-induced chemical processes for industrial applications on large scales. Besides, the associated societal strive for more stringent environmental regulations to battle some of the initial consequences of climate change have further fueled research and development of lower-carbon footprint chemical processes.

However, it remains challenging to translate novel photochemical processes from basic research or small-scale to large-scale implementation.

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Photochemical processes are generally integrated by a light source, a reaction medium, and a reactor. The light sources can be divided into natural (sunlight) or artificial, where the goal in terms of a “net-zero” chemical process is to harness the energy from the Sun directly. However, artificial light sources are increasingly becoming more and more efficient, mainly driven by the success of light-emitting diodes' (LED’s). High- efficiency light sources have been paramount in lowering the barrier for adopting some photochemical processes, given that it provides a solution to the intermittency of natural light. The light sources can then be further classified in terms of their wavelength range.

Monochromatic light sources are seldom preferred. Polychromatic light sources with an emission ranging from the UV (100-380 nm) to the visible range (380-780 nm) of the electromagnetic spectrum are most common. Industrially, the use of UV light has been more broadly adopted since at least the 1970’s.⁷That is mostly related to the access to free radical chemistry, considering that the photon energy matches the bond dissociation energy of most organic molecules.⁸

Table 1. 1 Bond dissociation energies for organic molecule bonds (adapted fromBlanksby et al.⁸)

Bond In eV/bond

Methyl C-H bond 4.550

Ethyl C−H bond 4.384

Isopropyl C−H bond 4.293

t-Butyl C−H bond 4.187

C−H bond α to amine 3.949

C−H bond α to ether 3.990

C−H bond α to ketone 4.163

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Vinyl C−H bond 4.809

Acetylenic C−H bond 5.763

Phenyl C−H bond 4.902

Allylic C−H bond 3.856

Benzylic C−H bond 3.907

Alkane C−C bond 3.60–3.90

Alkene C=C bond ~7.4

Alkyne C≡C triple bond ~10.0

A good number of photochemical transformations are accessible in the UV range, mainly isomerization and photolysis.^9–11 While UV light has critical practical advantages, the main drawback still is the dependency on lower efficiency, potentially hazardous ozone-generating sources. In addition, the UV light incident on the surface of the Earth from the Sun is insufficient, precluding competitive sunlight-driven processes.

Most recently, visible-light-initiated reactions have attracted attention thanks, in significant part, to the development of photo redox catalysis with organic and organometallic chromophores.^12–14 The use of visible light in photo redox catalysis has enabled access to important molecules that previously were difficult or even impossible to obtain through thermal chemistry.^15–18 Furthermore, this has triggered a substantial amount of research and excitement into the possibility of moving closer to solar-driven chemistry. These recent efforts reinforce the older ideal of performing visible-light-driven photocatalysis with metal oxides doped or functionalized to engineer their bandgap to

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harness lower energy photons. Our primary focus is on photoreactors and photocatalysts compatible with visible-light photochemistry.

The reactor design and materials are analyzed for some of the photochemical processes with the highest efficiencies. The effect of reactor design on efficiency has arguably received the most attention from researchers in the field. The latter stems arguably from the growing interest in photochemistry from researchers working in process engineering and intensification. Hence, a substantial fraction of the latest reports on the topic delves into the hydrodynamics analysis and the photon transfer as a function of reactor geometry.^19,20 Experiments and simulations have been utilized in several instances to investigate how these aspects impact the overall efficiency of the process.²¹ Importantly, this needs to be done for standard photocatalysts and formulations to isolate the reactor architecture's effect. Therefore, the connection to the intricacies of photochemistry are often lost. The success of continuous-flow processes for the intensification of thermal reactors and separations units has also contributed to the higher focus on reactor design. Microreactors have been previously reviewed more extensively, and the productivity is generally more difficult to increase with such low volumes even if operating under continuous flow. Photoreactors are available in varying volumes, but reactor designs operating with volumes of milli-liters or higher should be given more attention solely due to their higher productivity while keeping the advantages of microreactors intact in terms of conversions versus photon utilization. The materials used for photoreactor construction remain generally overlooked.^22,23 Fabrication of complex geometries at different scales poses a fair deal of complexities and increases the cost.

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We will briefly comment on the opportunities to explore materials science and engineering for photochemical process design.

1.2 Hypothesis

There have been numerous photocatalytic studies operated in relatively simpler photoreactors' designs in both continuous and batch mode, some being effective and others not due to the limitations in their inherent design. Our hypothesis is based on these studies as we aim to work with a novel complex static mixer design - a coiled flow inverter for continuous flow photochemistry. It has been proven to be a highly efficient reactor for multiple types of reactions owing to its superlative mixing by eddy generation. Suppose this superior design of reactor is operated with a visible light absorptive catalyst and complemented by a custom-made platform with a reflective surface. In that case, the photon efficiencies of the overall system should be significantly high as the reactor's design directly combats the problems of light attenuation by mixing and relatively higher productivity as operated at milli-scale. We also aim to use a highly efficient visible LED light source for driving our reaction, thereby operating the process at higher photonic and electrical efficiencies. We also hypothesize the usage of the SLA additive manufacturing technique for rapid prototyping of geometrically complex photoreactors, such as the CFI, by achieving higher transmissions equivalent/close to transparent materials.

1.3 Research Objective

Our long-term goal is to aid in adopting continuous-flow photochemical processes as a greener, safer, and more efficient alternative to thermally driven chemical processes.

It shall be achieved by having more flexibility in operations, by incorporating specifically

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tailored photocatalytic micro/milli-fluidic platforms and employing additive manufacturing technology.

1.3.1 Specific Objectives

With that in mind, we deal with the following specific objectives:

1) Scrutinizing the efficiency of a coiled flow inverter (CFI - static mixer) as a photoreactor in a highly flexible custom-made micro/milli-fluidic platform under visible light as a driving force. Functionalizing the metal oxide for making an effective visible light absorptive photocatalyst, to be characterized and utilized for the photo-degradation of a model pollutant, fluorescein. Implementation of online monitoring with fluorescence spectrometry for the reaction. Identification of a close to ideal light arrangement for CFI (relatively a complex design). Comparison of a CFI with other designs of photoreactors on a recently established metric (PSTY) for its efficiency.

2) Discussion of the possible interactions with the photocatalyst surface leading to any instabilities in the system.

3) To evaluate the usage of SLA 3d printing technique for making glass like transparent micro/milli-fluidics which can be utilized for rapid prototyping of various complex designs. Identify effect of different post-processing techniques on the subject.

1.4 Thesis Overview

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This document has been divided into different chapters, of which the summary is described below:

Chapter 2 presents the study of continuous flow photochemistry (photodegradation) in a coiled flow inverter design (static mixer) carried out on a milli-fluidic platform for effective light utilization and achieving higher photon efficiencies.

Chapter 3 presents the continuation of the study carried out in chapter 2. It emphasizes more on the different possibilities of model pollutant-photocatalyst interaction during the photodegradation leading to the settling of photocatalyst

Chapter 4 discusses the insights for achieving transparency in SLA 3d printed micro/milli-fluidics devices, complementing the study's preliminary results (yet not completed.)

Chapter 5 presents a summary of the key findings throughout the thesis work conducted and future direction and scope.

1.5 Theory and State of the Art

Here, in the state-of-the-art, photoreactor design is discussed as a mean to bring attention to some of the most challenging aspects precluding the adoption of photochemical processes, namely efficiency and productivity. We aim at complementing previous reviews on the subject by highlighting undermentioned aspects, such as the consideration of the photochemistry in addition to the hydrodynamic and irradiation schemes.^6,24,25 The majority of the present exemplary photoreactors were found to be proposed for the photo-oxidative degradation of organic molecules using semiconductor photocatalysis. The latter stems primarily from the older age of metal oxide photocatalysis

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as compared to photo redox catalysis and the principle that chain reactions are both thermodynamically and kinetically favorable, which aids the productivity of the overall process.

The reaction mediums that are covered in the discussion in this section are mostly multiphase. Given that most photoreactors were developed primarily for semiconductor photocatalysis, the media is inherently a combination of at least two distinct phases, typically liquid-solid. Reactors of this sort are now dominantly operated in a continuous flow to sustain competitive productivities (throughout) with low working volumes to exploit the principle of process intensification by down-scaling. From this point of view, the reactors may be classified in terms of whether the solid phase (photocatalyst) is immobilized or suspended in the reacting medium. Previous papers have reviewed the effect of catalyst immobilization for some of the most common metal oxides. However, the analysis has heavily centered around the hydro- and photon dynamics effects, without much connection to the complex heterogeneous chemistry occurring at the catalyst surface. Slurry flow reactors where an aqueous suspension of the photocatalyst is continuously irradiated has been documented to provide substantial benefits in terms of efficiency. This observation is analyzed herein to connect the hydro- and photon dynamics with the complexity of the Langmuir-Hinshelwood type reaction mechanisms, where many questions remain around the interaction between the organic molecules and the photocatalysts.

A unique comparative paradigm must be there for an evaluative study of various designs of photocatalytic reactors. A few criteria which have been employed more often in the past studies are useful and serve the purpose to a certain extent, but they can't be

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termed flawless. Certain limitations aren't considered in those criteria.^6,24 In section 1.5.1, are mentioned the definitions from the literature related to photochemistry and photochemical engineering which are widely used in the field and some of them are going to be useful in the current work in the chapters 2,3 and 4. A brief overview of the specific formulation that has been used to benchmark the latest photoreactors is also included.

1.5.1 Concepts of Photochemistry and Photochemical engineering 1.5.1.1 Quantum efficiency

One of the elementary criteria used for the comparison of the photochemical reactor designs is the quantum yield 𝛷. It is defined as the number of events of interest occurring per photon absorbed by the system.³ It can be expressed as below:

𝛷 [ 𝑚𝑜𝑙

𝑒𝑖𝑛𝑠𝑡𝑒𝑖𝑛] =𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑 𝑜𝑟 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝑓𝑜𝑟𝑚𝑒𝑑 [𝑚𝑜𝑙]

𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 [𝑒𝑖𝑛𝑠𝑡𝑒𝑖𝑛]

1.5.1.2 Photonic efficiency

Other one being photonic efficiency. It's expressed by, Ep= ^𝑅

𝐴× 𝑧 × 100 (1)

where 𝑧 refers to the number of transferred electrons per target molecule for degradation, R refers to the reaction rate in (mol L^-1 s^-1), 𝐴 refers to photon flux (mol L^-1 s^-1) and Ep

being the photonic efficiency (dimensionless). Quantum yield mainly exhibits the efficiency of reactor design for light utilization.^24,26,27

1.5.1.3 Apparent reaction rate

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Another essential criterion being the apparent reaction rate constant (kapp), which is indicative of the conversion rates. A comparison of conversion rates gives good enough insights on reactor design compatibility with the rate of reaction, but it misses out on giving the crucial information on throughput owing to its volume dependency furthermore not considering the variations in the absorbed photons and limitations in mass transfer with respect to different geometries. A suitable example²⁸ can be cited as a comparison between the two systems comprising of the same looped plug flow reactor (PFL) attached to different volume vessels. The system connected to the lower volume will yield a higher value of kapp then its counterpart despite having the same active area for the PFL. This criterion also depends on catalyst loading and light intensity.

Neither of them takes into consideration the productivity of the reactor nor electrical consumption. Therefore, two different designs of photoreactors may have the same value²⁸ as both the above-discussed criterion; nevertheless, they may have different productivity and also use different light sources.

1.5.1.4 Molar absorptivity

The molar absorptivity or molar attenuation coefficient relates to the measurement of how strongly a chemical species can attenuate light for a given wavelength. It is an inherent property of the species. The SI unit of molar attenuation coefficient is the square meter per mole (m²/mol), but in practice, quantities are usually expressed in terms of M⁻¹⋅cm⁻¹ or L⋅mol⁻¹⋅cm⁻¹ (the latter two units are both equal to 0.1 m²/mol). It is also known as molar extinction coefficient.

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1.5.1.5 Pathlength

The optical path length is given by the product of the geometric length (in m) of the path followed by light through a given system, and the refractive index of the medium through which it propagates.

1.5.1.6 Excited States

An excited state of a system (as in atoms, molecules, or nucleus) is any quantum state of the system that has a higher energy as compared to the ground state (also knowns as absolute minimum energy state). Excitation relates to an elevation in the energy above the baseline energy state.

1.5.1.7 Photoinduced electron transfer (PET)

An excited state generated by high energy photon absorption leading to an electron transfer process in which excited electron is transferred from the donor to acceptor is known as photoinduced electron transfer (PET). Charge separation (redox reaction) generated from PET leads to initiation of many chemical transformations.

1.5.1.8 Fluorescence

When the electromagnetic radiation is absorbed by a substance, it emits the lower energy radiation. This emission of lower energy wavelength (light) is known as Fluorescence and it belongs to the category of luminescence.

1.5.1.9 Metal oxide Semi-Conductor (MOS) Photocatalysis

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Metal-oxide semiconductors are usually characterized by their large bandgaps (>3.0 eV) (Figure 1.1). MOS are usually inexpensive, stable, safe, and abundant in availability.

Thereby, MOS are widely applied for variety of applications like photodegradation of organic pollutants in water and air, biosensing, microelectronics, optoelectronics, and storage. Their insoluble nature and higher chemical-photo-stabilities, it can be separated easily.

Figure 1. 1 Typical bandgap energies of some MOS presented and relationship between their band gap

and wavelength (figure adapted from Riente et al.⁷²)

1.5.1.10 Space Time

Time required to process one reactor volume of feed measured at specified conditions. Space time is the natural performance measure for flow reactors.

𝜏 = 1 𝑠 = 𝑉

𝜐₀ = 𝑉𝑜𝑙𝑢𝑚𝑒

𝑉𝑜𝑙𝑢𝑚𝑒𝑡𝑟𝑖𝑐 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒

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1.5.1.11 Static Mixers

A precision engineered device for the continuous mixing of fluid flow passing through it without any moving parts is known as static mixer. Generally static mixers are employed for liquid mixing, but the mixture of gas streams and multiphase systems can also be carried out.

1.5.1.12 Residence time

The total time spent by the fluidic element flowing inside a reactor volume is known as residence time. The residence time of a set of fluidic elements is measured in terms of the frequency distribution of the residence time in the set which is known as residence time distribution (RTD), or in terms of its average, known as mean residence time.

1.5.1.13 Reynolds number (Re)

The Reynolds number (Re) is the ratio of inertial forces to viscous forces within a fluid which is subjected to relative internal movement due to different fluid velocities. It helps to predict the flow patterns in the different fluid flow situations. At lower values (Re<2000) of Reynolds number, laminar flow pattern is dominated whereas at higher (Re>4000) values turbulent regime is dominated.

𝑅𝑒 =𝐼𝑛𝑒𝑟𝑡𝑖𝑎𝑙 𝑓𝑜𝑟𝑐𝑒𝑠 𝑉𝑖𝑠𝑐𝑜𝑢𝑠 𝑓𝑜𝑟𝑐𝑒𝑠 =𝑢𝐿

𝜐 = 𝜌𝑢𝐿 𝜇

(3)

where 𝑢 is fluid speed (m/s), 𝐿 is characteristic length (m) (or internal diameter in case of flow inside channels), 𝜌 is density (kg/m3), 𝜇 is dynamic viscosity (kg/m s), and 𝜐 is kinematic viscosity (m2/s).

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1.5.1.14 Dean number (De)

The Dean number (De) is a dimensionless group in the fluid mechanics, which specifically occurs while studying the flow patterns in the curved channels. It is given by,

𝐷𝑒 = 𝑅𝑒√ 𝐷 2𝑅_𝑐

(4)

where 𝑅𝑒 is Reynolds number, 𝐷 is diameter of the channel, and 𝑅_𝑐 is the radius of the curvature of the curved channel.

1.5.1.15 Damköhler number (Da)

The Damköhler numbers (Da) are dimensionless numbers used to relate the timescales of chemical reactions with the timescales of transport phenomena occurring in the system. We will define one of them for reacting system which consists of interphase mass transport, second Damköhler number 𝐷𝑎_|| which is given by,

𝐷𝑎||=𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒

𝐷𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 𝑟𝑎𝑡𝑒=𝑇𝑖𝑚𝑒_{𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛} 𝑇𝑖𝑚𝑒𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛

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1.5.1.16 Photocatalytic space-time yield (PSTY)

A new criterion of photocatalytic space-time yield (PSTY) was developed recently by researchers²⁴, taking into consideration the parameters discussed (1.5.1.1- 1.5.1.3) and adding the missing components by relating lamp power with the reactor design efficiency. PSTY is defined by the ratio of space-time yield (STY) to the power consumed (Lamp Power-LP in kW).

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STY can be found by the inverse of residence time (𝜏 in s^-1) of the fluid in the reactor.

For a reactor in a loop, a continuous stirred tank reactor (CSTR) model equation is used to predict the outlet concentration (CA) in mmol L^-1. For plug flow, photocatalytic reactors STY is given differently as per equation (4) below. Pseudo-first order rate constant (k) for the reaction is determined by fitting a straight line for a plot of [ ln (CA/CA0) vs. t] (time) and getting the slope for the same. Lamp power is scaled to the value enough for illuminating 1 m³ of the reactor volume, as seen in equation (5). LPstd refers to standardized lamp power (kW), P is the power of light source used in an experimental setup in (kW), and V refers to experimental reaction medium volume (m³).

𝐶_𝐴 = 𝐶_𝐴₀ 1 + 𝑘𝜏 (CSTR)

(6)

𝜏 = 𝐶_𝐴₀

𝐶_𝐴 − 1 𝑘

(Residence Time)

(7)

STYcstr= ¹

𝜏 = _𝐶𝐴0^𝑘

𝐶𝐴−1

(Space-time yield-CSTR)

(8)

STYpfr=¹

𝜏 = ^𝑘

𝑙𝑛 (^𝐶𝐴0 𝐶𝐴)

(Space-time yield-PFR)

(9)

LPstd= 𝑃 ×^{1 𝑚}³

𝑉𝑟

(Lamp Power)

(10)

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PSTY= ^STY

LPstd (11)

1.5.2 Common obstacles and possible solutions in Photocatalytic systems 1.5.2.1 Light Attenuation and Homogeneity

Photon absorption in the reaction medium is one of the most significant steps in the photocatalytic reaction mechanism. An unvaried distribution of photons throughout the reactor channels is critical for achieving higher conversion, selectivity, and yield for the target reaction.²⁹ Although radiation intensities do not remain consistent while being absorbed and decreases exponentially throughout the direction of absorption in the medium following the relation given by Beer-Lambert-Bouguer law.

𝐴 = −log₁₀𝑇 = log₁₀ 𝐼

𝐼₀ = 𝜀𝑐𝑙 (12)

where, absorbance (A) is related to molar attenuation coefficient (𝜀) or absorptivity of the attenuating species, concentration (c) of the attenuation species and optical path length (l) (T=Transmittance, 𝐼₀=Initial light intensity, 𝐼= light intensity after absorption).

Figure 1. 2 Relation of Distance into the medium and Mixing with % Transmittance

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The relation of transmittance and the varying channel diameters can be visualized in the Figure 1.2. With an increase in the diameter of the channels, light intensity decreases swiftly towards the center of the reactor.²⁹ Thus, lower diameter channels seem to be effective against the problem of light attenuation.³⁰ It is expected all photons to get absorbed and participate in reaction initiation, but in reality, not all of them are to end-up initiating the reaction. Light homogeneity is also one of the critical parameters,¹⁷ which is taken into consideration for achieving the best PSTY and higher conversions for the overall photoreaction system.

1.5.2.2 Mixing

Mixing plays a paramount role in any chemical reaction; the same is the case for photocatalytic reactions. Mixing helps in eradicating confined concentration gradients as seen in Figure 1.1, leading to the increase in the selectivity of products in wide range of reactor systems, especially in small scale reactors.³¹ Mixing in the case of the laminar region, i.e., layer on layer flow, which happens in micro- and milli-channels, is diffusion controlled. Smaller the diameter of the channels quicker, the uniformity in concentration can be reached and vice-versa. Therefore, micro- and milli-mixers are helpful for micro- and milli-small scale systems to improve the mixing time and to make the process more efficient.^32,33

1.5.2.3 Low productivity

Since the inception of the idea behind photochemical reactions, chemists have been employing the milli/centi-scale (ID > 1 cm) batch reactor systems where they were facing

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problems with the decrease in light intensities owing to limitations of Beer-Lambert law.

The best possible solution to the problem mentioned above was scaling down to micro- scale processes (ID < 1mm). Although micro-scale processes allow higher and homogenized photon flux, higher conversions, shorter reaction times, improved heat/mass transfers, and lesser unwanted side-product generation, they also undergo with the problem of lower throughputs.

There have been some studies in numbering up^34,35 the microscale photoreactor systems, which are still not being capable enough to compete with the existing large- scale processes for the same end applications due to extremely less throughput per day.

Milli-scale continuous processes can be a real viable option in terms of productivity, and the problems faced by larger reactors can be solved by the selection of suitable designs like a static mixer³⁶ which reduces the light attenuation problem by swirling liquid to the well-lit zone and continue the same until the outlet.

1.5.2.4 Photocatalyst separation/recovery

Photocatalytic systems are either homogeneous or heterogeneous. Either of them uses photocatalysts, which are in solid or liquid form, but they are supposed to be recovered and replenished after every intended reaction. This recovery issue is one of the critical challenges coming against photocatalysis to make it functionally large scale, as many photocatalysts contain costly noble metal particles or metallic compounds (e.g., Pt, Au, Ag, Ru, and others). Novel separation methods are to be implemented for recovering and activating the catalyst for a new reaction, as reported in some studies³⁷ like usage of magnetic nanoparticles. Overall, it is complicated for continuous flow

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photochemical processes owing to their small sizes, and this field of separation/recovery needs to be focused more for achieving an efficient methodology for the same.

1.5.2.5 Efficiencies/Effectivity of light sources used

Historically low/medium pressure mercury discharge-based UV light sources and incandescent halogen light sources have played an important role at the core of photochemistry-based applications. UV spectrum is highly effective when compared to visible spectrum purely owing to its higher energy potential comparatively. Solar spectrum also consists of UV but its only 3~5% of the total spectrum whereas 42~43% is visible and the rest in infrared. Although the above mentioned conventional light sources are well assimilated by the market, still they have drawbacks of using toxic materials, high voltage requirements along with higher ignition pulses, thereby making it less adaptable. Newer technologies like LEDs utilize inherently safer materials along with lower operating voltages and comparatively smaller dimensions making it highly flexible and less frail.

One of the other biggest challenge with the older conventional light sources is their nearly fixed or less tunable radiant power (in W/cm²), which is not the case for LEDs, where it is quite easy to control/vary and achieve the high values of radiant power with the current.

Thus, LEDs can be costly for initial investments but give flexibility with various complex geometries; high radiant power and smaller form factor can be helpful in making of highly efficient and versatile photocatalytic reactor systems.

Table 1. 2 The comparison between various designs of photocatalytic reactors with respect to their PSTY values

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Design Principle

Reactor Type

V(L) kapp (min^-1)

STY (m³ day^-1 m^-3 reactor)

LPstd (kW) PSTY (m³ day^-1 m^-3 reactor kW^-1)

Refs.

Slurry reactors

MEM (3) 12 N/A 2.88×10³ 5.12×10² 5.63×10⁰ ³⁸

CMSTR(1) 2 0.107 1.54×10⁻¹ 4×10⁰ 3.86×10⁻² ³⁹

CFI 0.025 0.17 3.203×10¹ 1.08×10³ 2.97×10⁻² ⁴⁰

MEM (1) 1 0.198 2.86×10⁻¹ 3×10¹ 9.52×10⁻³ ⁴¹

EISR (1) 1 0.003 6.80×10⁻¹ 1.5×10² 4.54×10⁻³ ⁴²

RAR (1) 1.1 0.077 1.11×10⁻¹ 3.63×10¹ 3.05×10⁻³ ⁴³

MEM (2) 1.2 0.113 1.63×10⁻¹ 8.33×10¹ 1.95×10⁻³ ⁴⁴

ARAR 3.9 0.02 2.88×10⁻² 3.07×10¹ 9.37×10⁻⁴ ⁴⁵

ASAR 3.9 0.019 2.83×10⁻² 3.07×10¹ 9.18×10⁻⁴ ⁴⁵

AR (1) 1.53 0.016 2.31×10⁻² 3.02×10¹ 7.63×10⁻⁴ ⁴⁶

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SAR 3.90 0.005 7.50×10⁻³ 3.07×10¹ 2.44×10⁻⁴ ⁴⁵

EISR (2) 0.25 0.035 5.05×10⁻² 2.67×10² 1.88×10⁻⁴ ⁴⁷

EISR (3) 0.75 0.053 7.68×10⁻³ 1.66×10² 4.61×10⁻⁵ ⁴⁸ Immobilized

catalyst reactors

TPR 0.05 0.818 1.71×10² 2.72×10² 6.28×10⁻¹ ⁴⁹

SDR (1) 10 0.031 4.47×10⁻² 3×10⁰ 1.49×10⁻² ⁵⁰

PBR (1) 0.30 0.005 1.13×10⁰ 2.66×10² 4.23×10⁻³ ⁵¹

FPR (1) 0.34 0.021 3.03×10⁻² 9.06×10⁰ 3.34×10⁻³ ²⁸

PBR (2) 0.30 0.002 5.42×10⁻¹ 2.66×10² 2.03×10⁻³ ⁵¹

RAR (2) 1.10 0.044 6.34×10⁻² 3.63×10¹ 1.74×10⁻³ ⁴³

FPR (2) 3.00 0.025 3.60×10⁻² 3.50×10¹ 1.03×10⁻³ ⁵²

OFMR(1) 0.90 0.16 2.31×10⁻¹ 5.55×10² 4.15×10⁻⁴ ⁵³

SDR (2) 0.84 0.024 3.49×10⁻² 9.68×10¹ 3.60×10⁻⁴ ⁵⁴

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CDR 1.25 0.019 2.77×10⁻² 9.60×10¹ 2.88×10⁻⁴ ⁵⁵

MR (2) 3.20×10⁻⁵ 0.05 1.04×10¹ 3.81×10⁴ 2.74×10⁻⁴ ⁵⁶

MR (4) 1.14×10⁻⁴ 0.47 1.00×10² 1.75×10⁶ 5.70×10⁻⁵ ⁵⁷

MR (1) 1.50×10⁻⁶ 18.00 3.76×10³ 8×10⁷ 4.70×10⁻⁵ ⁵⁸

MR (3) 3.25×10⁻⁵ 0.73 1.53×10² 3.69×10⁶ 4.16×10⁻⁵ ⁵⁹

OFR (1) 0.30 0.001 1.59×10⁻³ 1.66×10³ 9.51×10⁻⁷ ⁶⁰

1.5.3 Highly effective designs as per PSTY for slurry and immobilized catalyst reactors Here we discuss the top three designs in each category i.e., slurry-based system and immobilized catalyst systems, as per PSTY values. A log-log plot of PSTY vs STY can be seen in Figure 1.3 and top three designs in both slurry and immobilized catalyst reactor category are marked with their names.

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Figure 1. 3 Plot of PSTY vs STY for various designs, highlighting the top three designs in Slurry and Immobilized reactor category

1.5.3.1 Slurry reactors

1.5.3.1.1 Photocatalytic membrane reactor (MEM)

A membrane has been employed for many applications such as, in purification or separation processes like dialysis,⁶¹ carrying out reactions by providing huge surface area as in zeolites,⁶² and is also modified to assign custom properties like photoactivity,⁶³ which justifies there have been lot of experimentation employing membranes for the various systems.

The MEM pilot reactor technology³⁸ patented by Photo-CatTM system discussed here employs membrane as a filter/separator unit, which is aimed for catalyst recovery and recycles it back to the reactor inlet like a purge line.

As can be seen in the figure 1.4, the system consists of prefiltration units, having bag/cartridge filters with a nominal pore size of 10µm. The geometry employed in this case is thin-film reactor (exact image not available due to patented technology), which is adjoined with more significant mixing owing to plug flow pattern, a containment sleeve, and UV light source (185~254 nm) arrangement. There is a catalyst recovery unit comprising of the crossflow ceramic membrane, which selectively stops TiO2 nanoparticle photocatalyst, while allowing the water to pass through the membrane. To prevent clogging of the membrane, after every 60s a backflush is carried out, which carries catalyst again to the entry of the reactor via the recycle line as seen in the schematic. The pattern of recycling, and reuse of photocatalyst along with better mixing makes this combination one of the best in slurry reactors as it checks all the necessary requirements.

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Thin-film reactor design deals with better light attenuation and homogeneity, plug flow regime assists in achieving better mixing, semi-continuous (because of backflush cycle) mode of operation has higher productivity (34,560 lit/day) for a pilot scale, and one of the most critical aspects of photocatalyst recovery is taken care off by catalyst recovery unit.

Figure 1. 4Schematic of photocatalytic membrane reactor system with visualization of crossflow ceramic membrane (Image adapted from Benotti et al.³⁸,Llorca et al.^,64)

1.5.3.1.2 Continuous magnetically stirred tank reactor (CMSTR)

Generally, in the chemical industry at small to moderate scale unit operations;

usually, they incorporate continuous agitated stirred tank reactors, often known as CSTR or MFR (mixed flow reactors). They can also be tailored for batch scale recirculation operations for complete recirculation like a recycle CSTR having recirculation ratio to be infinite. The reactor considered here is similar, having batch recirculation behavior. It

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comprised of a cylindrical glass structure comprising of magnetic stirrer and equipped with a low-pressure UV light source at the center of the container, jacketed by a protective glass for avoiding contact from water. As the light source is at the center of the container, almost all of the volume of the liquid is irradiated uniformly, keeping light intensity to be reasonably high for the given study at 10 mW/cm² and spectral emission at 366nm.

Photon flux was controlled by portable photo radiometer arrangement attached to the glass wall.³⁹

Highlights of this design are special provision of bubbling air into the liquid system after every 10 mins, which avoids oxygen deprivation and aids in the generation of reactive oxygen species (ROS) (i.e., hydroxyl/superoxide/peroxide radicals) and ceases the recombination of separated electrons with the generated holes. Oxygen works as an electron dumping ground for photodegradation processes. While stirring aids in keeping uniformity in the suspension, a water-cooling jacket helps to keep uniformity in temperature throughout the process run (can be visualized in Figure 1.5). This unique yet straightforward stirred tank design tackles the problems by providing homogeneity in light distribution by its annular centric design and takes care of oxygen nourishment in the recirculated liquid for the generation of ROS along with better mixing.

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Figure 1. 5Schematic of continuous magnetic stirred tank reactor (Image adapted from Vela et al.³⁹)

1.5.3.1.3 Coiled Flow Inverter (CFI-Static Mixer design)

It is one of the well-known static mixer designs developed³⁶ around 1984, and only until sometime back⁴⁰ in the starting of 2020, it was discovered to be an effective slurry type reactor design for photochemical reactions because of higher catalyst loading availability and enhanced mixing it entails to the system. CFI's have been successfully employed in many nonphotochemical applications; extraction, mixing, heat exchangers, and reactors being some of them.

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Figure 1. 6CFI in the categorization of Micromixers (Image adapted from Vural Gürsel et al. ⁶⁵) along with the visualization of flow inversion due to bending in a coil (Image adapted fromVashisth,S et al.⁶⁶)

In the helicoidal coil of CFI design, centrifugal forces on the high velocity fluid flowing at center, resulting in an unstable stratification making the high velocity fluid deflect outwards along the pipe bend, leading to a formation of two counter-rotating vortices, also known as dean vortices. To avoid the flow to reach full developed flow regime, periodic perturbations are introduced by 90-degree bends which inverts the axis (can be visualized in figure 1.6) of the dean flow by 90 degree leading to chaotic advection.³⁶ This unique flow patterns have a significant effect on heat and mass transfer capabilities.^36,66 As

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discussed by Saxena et al.³⁶, bringing abrupt changes in the direction of centrifugal forces (in helical coils with bends) are more effective in narrowing down the residence time distribution (RTD) in the system than the gradual changes (in helical coils without bends).

Although, it is worth noticing that RTD is sensitive to the number of parameters, namely, the number of bends, the spacing between bends, the angle between different arms of the helix, and dean number.³⁶

It's a daunting task to irradiate the CFI design uniformly.⁴⁰ A proper arrangement of the light source is necessary to avoid any power wastage and getting the best homogeneity with a highly efficient light source. Overall, this design tackles the problem of mixing and productivity because of its inherent design, which can process higher throughputs of fluids, although it needs to be coupled with a catalyst recovery system and optimum flow rates for avoiding settling of catalyst for making it highly efficient, as will be discussed later.

1.5.3.2 Immobilized catalyst reactors 1.5.3.2.1 Translucent packed bed reactor

Many studies on immobilized catalyst reactors for various applications ranging from chemical synthesis to toxic/waste chemical degradation over a range of immobilized catalysts (enzymes/metal oxides).^67–70 The reactor design taken into consideration for this discussion is unique where the main highlight was scale up of the design equivalent to the multiple microreactors in lieu of the numbering up strategy, thereby enhancing the surface area of the reaction and achieve mixing by flow distribution. It can be seen in the Figure 1.6, the surface area for catalyst immobilization is provided by small uniformly

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sized transparent borosilicate glass spheres. Although spherical shape has the lowest surface area and surface-to-volume ratios amongst the other ordered shapes, it easily self assembles to an ordered stack arrangement and relatively straightforward for surface coat treatment. They are further treated with a coat of the TiO2 P25 layer. The reactor's geometry is relatively simple, which is a compilation of inlet, middle, and outlet sections, as seen in Figure 1.7. The inlet section is filled with uncoated 3 mm diameter glass beads, which act as micro-scale distributors and divert the liquid to the coated active surface stacked in the middle section between two parallel flat glass surfaces. The height of the stacked beads is the main reactive structure/surface, which is coupled with the height of the light source used for the reaction. Being the essential part of the reaction, the base structure of the bed which is made of stacked glass beads should be transparent to the emission spectra of the light source.⁴⁹

Porosity of an ideally stacked packing is significantly less leading to sub-mm scale hydraulic diameters.⁴⁹ Any shift away from ideal packing arrangement shows an increase in hydraulic diameters leading to some cases having higher hydraulic diameters than millimetric scale depending on the diameter of the beads used.⁴⁹ The surface-to-volume ratio is dependent on the diameter of the beads and structural porosity and it was shown that with a decrease in the diameter of the beads for a specific fixed value of structural porosity leads to a significant increase in the surface-to-volume ratio, which is in the order of the micro-scale reactors and slurry-based reactors along with a satisfactory value of catalyst loading.

the oxidative degradation of fluorescein induced by visible light in the presence of ZnO-APTMS-Au micro/nano-particles in aqueous suspension

Instituto Tecnológico y de Estudios Superiores de Monterrey

School of Engineering and Sciences

Lab-scale modular platform to study coiled-flow inverters (CFI) as candidates for continuous-flow photoreactor units: A case study based on

the oxidative degradation of fluorescein induced by visible light in the presence of ZnO-APTMS-Au micro/nano-particles in aqueous suspension

Chinmay Pramodkumar Tiwari

Lab-scale modular platform to study coiled-flow inverters (CFI) as candidates for continuous-flow photoreactor units: A case study based on the oxidative degradation of fluorescein induced by visible

light in the presence of ZnO-APTMS-Au micro/nano-particles in aqueous suspension

by

Chinmay Pramodkumar Tiwari

Abstract

List of Figures

List of Tables

Contents

Chapter 1

1. Introduction