LIGHTBEST From 254 nm to 365 nm—Why Does the Photochlorination of Cyclohexane Require Re-selecting the UV Light Source_1.pdf

—Analysis of UV light source and reaction efficiency in photochlorination under corrosive HCl/Cl₂ media conditions

                                                                                          Lightbest  Leo

Abstract: This paper analyzes the technology of a typical industrial photochlorination process: the reaction medium mainly includes cyclohexane, chlorocyclohexane, hydrogen chloride, and trace amounts of chlorine gas, with a quartz lamp sleeve installed outside the UV light source. The paper discusses suitable UV light source schemes for industrial continuous photochlorination reactors from aspects such as Cl₂ photolysis mechanism, wavelength matching, comparison of 254nm/330–365nm light sources, quartz sleeve, temperature effect, optical path length, and photon utilization efficiency.

1. Photochlorination does not "illuminate the reactants," but rather utilizes UV light to produce Cl· free radicals.

The photochlorination of cyclohexane is not traditional UV sterilization, but rather a free radical chain reaction. UV light first acts on molecular chlorine, causing Cl₂ to undergo photolysis, generating highly reactive chlorine radicals (Cl·).

Cl₂ + hν → 2Cl·

Subsequently, Cl· removes a hydrogen atom from cyclohexane to generate a cyclohexyl radical; the cyclohexyl radical then reacts with Cl₂ to generate chlorocyclohexane, while simultaneously regenerating Cl·, thus forming a chain reaction. Therefore, the key issue in UV light source design is not simply pursuing electrical power, but rather ensuring that as many effective photons as possible are absorbed by Cl₂ and converted into effective radical reactions.

2. Why is the 330–365nm range worth focusing on?

Publicly available spectral data show that Cl₂ exhibits significant ultraviolet absorption in the range of approximately 250–450nm, with particularly strong absorption around 330–335nm. This implies that the area around 330nm provides a good spectral match for the single target of "Cl₂ photolysis".

3. 254nm is not unusable, but it may not be the optimal first choice.

Low-pressure mercury lamps at 254nm are a mature industrial UV band, so from the perspective of equipment maturity and supply chain, 254nm can certainly be used for photochlorination experiments. However, the goal of this project is the photolysis of Cl₂, not microbial inactivation. The photon energy at 254nm is higher than that at 365nm; moreover, the actual reaction efficiency also depends on the absorption of Cl₂ in the reaction solution, the thickness of the liquid layer, gas-liquid mass transfer, and side reactions. For larger industrial reactors, simply increasing the power of the 254nm lamp does not necessarily result in a proportional increase in production capacity.

Therefore, a more reasonable engineering approach is to use 254nm as a control and conduct experiments under the same conditions as 330–340nm and 365nm, rather than directly assuming that 254nm is necessarily the optimal solution.

4. Why is 185nm not recommended?

185nm is often seen alongside 254nm in the spectrum of some low-pressure mercury lamps, but this process already includes Cl₂, HCl, and organic solvent systems, making it unnecessary to introduce an additional photochemical pathway due to 185nm. 185nm can promote ozone formation from oxygen and may enhance the photolysis of organic matter and material aging. Therefore, in engineering practice, it is recommended to use ozone-free UV lamps and selectively suppress 185nm transmission through lamp tube materials or glass/quartz.

5. Quartz lamp sleeve: The orientation is correct, but attention must be paid to the material and transmittance.

In corrosive organic chlorination media, physically isolating the UV lamp from the reaction medium is a reasonable engineering design. A quartz sleeve can isolate the lamp tube, lamp electrodes, and electrical components from the reaction medium while maintaining good UV transmittance. Historically, photochlorination devices also featured similar structures where the UV lamp was placed inside a quartz tube and the reactants flowed outside the tube.

However, the mechanical strength of quartz sleeves should not be the sole consideration. Transmittance at the target wavelength, wall thickness, OH content, UV aging, thermal shock, sealing structure, and reliability under long-term contact with HCl/Cl₂ environments should also be verified.

6. What effect does the high temperature inside the reactor have on the light source?

The temperature inside a typical reaction vessel is around 50–55°C , making cooling impossible. This temperature itself doesn't necessarily prevent the UV light source from operating, but different lamp types require different thermal management methods. Ordinary low-pressure mercury lamps are quite sensitive to lamp wall temperature and mercury vapor pressure; when the temperature of the lamp tube and its surroundings deviates from the design operating point, the 254nm output may decrease. If a low-pressure mercury lamp is used, the actual operating temperature of the lamp tube, mercury vapor pressure, whether a mercury amalgam is required, and the temperature difference between the inside and outside of the sleeve should be carefully evaluated .

LIGHTBEST's medium-pressure mercury lamps have a wide spectrum and high power per unit, making them particularly attractive in industrial settings that require high light intensity and compact reactors.

7. How to select from the three types of candidate light sources?

8. How exactly should photochlorination efficiency be defined?

Industrial design cannot simply rely on empirical calculations like "how many tons of cyclohexane can a 300W lamp process." The true determinants of production capacity are effective photon absorption and reaction kinetics. Overall efficiency can be broken down into: the efficiency of the lamp's electrical power to effective UV light power, the proportion of UV light absorbed by Cl₂, the photolysis quantum efficiency of Cl₂, and the efficiency of free radicals ultimately entering the target chlorination reaction.

ηoverall ≈ ηlamp × etaabs × ΦCl₂ × ηreaction

ηabs is particularly critical. If the Cl₂ concentration is high, light may be largely absorbed in the very thin liquid layer near the quartz sleeve; in this case, further increasing the lamp power may not necessarily increase the conversion rate of the entire reactor proportionally.

9. Optical path length is more important than simply increasing lamp power.

For this type of high-absorption photochemical system, LIGHTBEST recommends prioritizing reactors with thin liquid layers, multiple lamps, and enhanced mixing, rather than simply increasing the reactor diameter. For example, LIGHTBEST ... The ring structure of "UV lamp - quartz sleeve - thin-layer reaction liquid" allows the effective optical path to be kept within a small range by controlling the distance between the lamp and the reactor wall.

If a larger processing capacity is required, multiple LIGHTBEST parallel lamps can be used to allow the reaction solution to be repeatedly irradiated with UV light in multiple short optical path regions, thereby reducing dark areas and increasing the light utilization rate per unit volume.

10. How should industrial experiments be conducted?

LIGHTBEST recommends establishing a unified "light source-reaction-product" evaluation platform, rather than simply measuring UV irradiance. At least the following inputs and outputs should be recorded:

Inputs: cyclohexane flow rate, Cl₂ flow rate or concentration, HCl concentration, temperature, pressure, UV power, target wavelength, optical path length, and residence time.

Outputs: Cl₂ conversion, cyclohexyl chloride/chlorocyclohexane yield, monochlorine selectivity, dichloride and polychlorinated byproducts, unreacted cyclohexane, and HCl production.

LIGHTBEST recommends adding: online or offline GC analysis to distinguish between target products and byproducts; and recording the decay of lamp UV output over time.

11. It is recommended to use STY to compare different UV light sources.

For industrial-scale selection, a very practical indicator is the target product output per unit of UV energy consumption:

STY = kg chlorocyclohexane / kW·h UV

Under identical cyclohexane flow rate, Cl₂ feed, temperature, pressure, optical path length, and residence time, tests were conducted at 254nm, 330–340nm, 365nm, and with a medium-pressure mercury lamp. The Cl₂ conversion, monochloride selectivity, and STY were then compared. This approach determines which light source is truly suitable for industrial applications, rather than relying solely on the rated wattage of the UV lamp.

12. Recommendations for Lightbest’s customers

LIGHTBEST is involved in the light source development for this project , it is recommended to first establish a "three-path parallel" experimental platform:

1. Route A: High-power medium-pressure mercury lamp, utilizing a 250–450nm broad spectrum to cover the Cl₂ absorption band.

2. Route B: 365nm high-power UVA mercury lamp, focusing on verifying industrialization cost, lifespan, and unit energy consumption.

3. Route C: 330–340nm narrowband light source, focusing on verifying quantum utilization efficiency in the Cl₂ strong absorption band.

254nm low-pressure mercury lamps are recommended as a baseline control; 185nm lamps are recommended to be excluded. The final selection should be determined by the measured Cl₂ conversion rate, target product selectivity, STY, lamp life, and long-term reliability of the quartz sleeve.

Safety and Engineering Specifications

This article discusses the technical route for photochemical reactors and UV light sources. Cl₂, HCl, and organochlorides pose corrosive, toxic, and/or flammable risks; therefore, actual equipment must be engineered according to applicable pressure vessel, explosion-proof, electrical, leak detection, exhaust gas treatment, material compatibility, and chemical safety regulations. The light source selection and efficiency relationships discussed in this article are part of a technical analysis framework and cannot replace a complete process safety assessment and pilot-scale verification.