Design - Scale - up of Photo Chemical Reactor

Hello All, 

Hope everyone is doing good. 

Welcome back to PharmaCalculations.com! In today’s post, we dive into the fascinating world of Photochemical Reactor Design — a topic gaining immense attention in modern pharmaceutical manufacturing due to its green, efficient, and highly selective nature. This post was drafted for more than an year. During my course work in PhD i got an opportunity to go through the basics of Beer - Lambert Law and its application in Photo chemistry and i've starting drafting the post since then by adding the scale up part.

We’ll start by understanding the working principle, then design a lab-scale system with detailed calculations, and finally demonstrate how to scale up a photochemical reactor correctly — based on absorbance, not just residence time.

What is a Photochemical Reactor?

A photochemical reactor uses light energy (typically UV or visible) to activate molecules, driving chemical reactions that may not proceed thermally. Photons excite electrons in molecules, leading to bond cleavage, rearrangement, or radical initiation.

Common applications in pharma:

• Photochlorination or bromination

• UV-induced oxidation

• [2+2] cycloadditions

• Photoredox catalysis using visible light

Why is absorbance more important than residence time in photochemical scale-up?

Because the reaction is light-driven. Maintaining photon absorption ensures reaction efficiency, unlike thermal reactors where time dominates.

What is quantum yield in photochemistry?

It’s the number of molecules transformed per photon absorbed. It shows how effective a photon is in driving reaction.

How do you ensure complete light absorption?

By ensuring high absorbance (A ≥ 2) through suitable concentration, path length, and light wavelength.

Can visible light be used in photochemistry?

Yes, especially with photocatalysts like Ru(bpy)₃²⁺ or eosin Y. Reactions include C–C coupling, oxidations, and rearrangements.

Why are annular reactors common in lab systems?

They allow tight control of optical path (1–2 cm), surrounding the lamp and maximizing absorption.

What happens if light penetrates too deeply?

It indicates low absorbance. Most photons pass through without being absorbed, resulting in poor reaction efficiency.

What are typical residence times for photoreactors?

Generally 30–60 minutes, depending on photon flux, conversion targets, and reaction order.

How do we handle heat generation from lamps?

Use cooling jackets or flow reactors with thin films. LEDs are preferred for low heat load.

What is photon flux and how is it measured?

Photon flux is the number of photons per unit area per second. It’s measured in einstein/cm²·s using actinometry or radiometry.

Can I use batch mode for photochemical reactions?

Yes, but continuous flow offers better light penetration, heat control, and scalability.

Which geometry is ideal for scale-up?

Thin-film flow cells, multiple annular tubes, or LED panel-based systems that maintain short path lengths and uniform illumination.

Now, lets get into the principle, design and scale-up part.

Basic Principle

The rate of a photochemical reaction depends on light absorption, governed by:

$$r = \phi \cdot I \cdot \epsilon \cdot C_A$$

Where:

• $r$ = rate of reaction (mol/L·s)

• $\phi$ = quantum yield (mol/einstein)

• $I$ = photon flux (einstein/cm²·s)

• $\epsilon$ = molar absorptivity (L/mol·cm)

• $C_A$ = concentration of reactant (mol/L)

Now, i'll assume some laboratory inputs for designing a Photo-chemical reactor

Parameter	Value
Reaction	Photochlorination of toluene
Quantum yield $\phi$	0.5 mol/einstein
Wavelength	365 nm (UV)
Photon flux $I$	4 × 10⁻⁶ einstein/cm²·s
Molar absorptivity $\epsilon$	150 L/mol·cm
Concentration $C_A$	0.2 mol/L
Optical path length $l$	1.0 cm
Target conversion	80%
Flowrate	30 mL/min = 0.03 L/min

Step-by-Step Lab Reactor Design

Step - 1: Calculate Absorbance

$$A = \epsilon \cdot C_A \cdot l = 150 \cdot 0.2 \cdot 1 = 30$$
$$\% \text{Light absorbed} = 1 - 10^{-30} \approx 100\%$$

Nearly all incident photons are absorbed — ideal for efficiency.

Step - 2: Calculate Reaction Rate

$$r=\varphi \cdot I\cdot ϵ\cdot C_A=0.5\cdot 4\times {10}^{-6}\cdot 150\cdot 0.2=6\times {10}^{-5}\text{ mol/L.}\text{<span style="color: black; font-weight: bold;">s</span><br><span style="color: black; font-weight: bold;"><br></span>}$$

Step - 3: Calculate Required Residence Time

$$\tau = \frac{C_{A0} \cdot X}{r} = \frac{0.2 \cdot 0.8}{6 \times 10^{-5}} = 2666.67\ \text{s} = 44.4\ \text{min}$$

Step - 4: Reactor Volume

$$V = \tau \cdot v_0 = 44.4 \cdot 0.03 = 1.33\ \text{L}$$

Lab reactor volume = 1.33 L

Hope, the design part is clear for everyone.

Now let's jump into the scale-up part and do that in three simple steps

SCALE - UP 

Let’s scale the process from 0.03 L/min to 5 L/min, i.e., 167× flow-rate increase.
Do not scale volume directly. Instead, preserve absorbance instead of residence time as the absorbance is the driving force for the scale - up.

Step - 1: Maintain Absorbance

$$A = \epsilon \cdot C_A \cdot l = 30 \Rightarrow \text{Keep } l = 1.0 \text{ cm}, \epsilon = 150, C_A = 0.2$$

Step - 2: Calculate Required Reactor Volume

$$V_{\text{plant}} = 5 \cdot 44.4 = 222\ \text{L}$$

Step - 3: Determine Required Irradiated Area

$$V = A_{\text{irr}} \cdot l \Rightarrow A_{\text{irr}} = \frac{222}{1\ \text{cm}} = 22200\ \text{cm}^2 = 2.22\ \text{m}^2$$

Required irradiated surface area = 2.22 m²

That's it .....!!

Hope the scale-up part is clear for everyone.

If any queries, feel free to comment or reach us at pharmacalc823@gmail.com

Comments are most appreciated .........!!!!

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About The Author

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Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a tech geek by passion, and a chemical process engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
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