# RFLF VOC Kinetics Lab - Methodology

## 1. Scope and evidence basis

This case study restructures the experimental evidence reported for FAPESP project 2020/09004-2, "Fotocatálise em leito fluidizado de alta velocidade: obtenção dos parâmetros cinéticos dos poluentes de interesse". The structured files do not add new laboratory measurements. Smooth model curves are reconstructed from the equations and fitted constants reported in the final scientific report.

## 2. Photocatalyst preparation and qualification

The supported photocatalyst contains 20 wt% TiO2 on SiO2. The synthesis sequence is:

1. Combine 8.5 mL TTIP with 1 M HNO3.
2. Stir until transparent.
3. Add NaOH to pH 2.
4. Add the SiO2 support.
5. Agitate the suspension.
6. Add NaOH to pH 3.
7. Rest in dialysis tubing; repeat dialysis when required.
8. Confirm supernatant pH above 5.
9. Filter, dry, calcine at 450 C for 1 h and mechanically sieve.

The final material is reported as 98% anatase and 2% rutile, with a specific surface area of 430 m2 g-1, a particle-size interval of 40-200 um and a Sauter mean diameter d[3,2] of 59 um. The 460 m2 g-1 value is retained only as support/context information.

## 3. Photocatalytic mechanism represented on the website

The mechanism diagram follows the three stages described in the report:

- photon absorption and generation of an electron-hole pair;
- charge separation and migration to reactive surface sites;
- surface reduction and oxidation reactions.

For TiO2, a 254 nm UVC photon exceeds the approximately 3.2 eV band gap and promotes an electron from the valence band (BV/HOMO) to the conduction band (BC/LUMO). Electrons participate in reduction routes involving acceptors such as O2, while holes oxidize adsorbed VOCs directly or participate in routes involving water-derived hydroxyl species. The experiments quantify disappearance of the parent VOC; complete mineralisation was not demonstrated.

## 4. Experimental unit

The RFLF is an annular high-velocity circulating fluidized-bed photoreactor comprising:

- a 50 mm internal-diameter borosilicate outer tube;
- a concentric 30 mm external-diameter quartz tube;
- total height of 850 mm;
- a Philips TUV36 4P T5 HO germicidal UVC lamp, 75 W, 254 nm;
- approximately 1 L irradiated annular volume;
- a cyclone, solids reservoir and screw feeder for solids recirculation.

The operating setpoint is Q = 23.8 L min-1 and Gs = 0.8 kg m-2 s-1. The solids reservoir is loaded with approximately 200 g, whereas the irradiated reactor zone contains approximately 3 g under the selected operating conditions.

The gas-conditioning train includes a compressor, heat exchanger, adsorption dryer and rotameter. Separate water and VOC bubbler branches control humidity and pollutant concentration. The VOC bubbler is held at 5 C and relative humidity is maintained between 45% and 55%.

## 5. Sampling and steady-state acceptance

A low-flow peristaltic pump draws approximately 100 mL min-1 from the inlet or outlet. An MSU-8V4S-RS48 valve relay alternates the sampling position and routes the gas to GC-PID/TID.

The programmed sequence is two inlet samples followed by four outlet samples. A run is closed only after three cycles show stable mean inlet and outlet concentrations.

UVC is switched on only after:

- outlet concentration C is approximately equal to inlet concentration C0 with the lamp off, indicating adsorption saturation;
- reactor pressure drop Delta P is stable, indicating stable solids holdup;
- relative humidity is stable within 45-55%.

## 6. Experimental dataset

The completed matrix contains 41 experiments:

- 40 concentration-conversion observations used for kinetic fitting;
- 10 observations each for n-hexane, n-pentane, benzene and toluene;
- one successful long-duration butanone experiment.

Direct UVC photolysis was negligible for the four fitted VOCs under the study conditions. Butanone showed only about 5% removal under UVC alone at a doubled residence time of 5.4 s, so photolysis was not material to the photocatalytic result.

## 7. Kinetic model

The model combines a plug-flow material balance with a single-component Langmuir-Hinshelwood rate law:

```text
Q dC/dm = -r_m
r_m = (k Ka C) / (1 + Ka C)
X = 1 - C/C0
```

Where Q is gas volumetric flow, C0 is inlet concentration, C is local/outlet concentration, m is catalyst mass in the irradiated zone, r_m is mass-specific reaction rate, k is the surface-reaction constant and Ka is the adsorption constant.

Parameters were estimated separately for each fitted VOC by nonlinear least squares using the Levenberg-Marquardt algorithm and ten observations per pollutant.

| Pollutant | k (mg gcat-1 h-1) | Ka (m3 mg-1) |
|---|---:|---:|
| n-hexane | 13.7 | 0.0204 |
| n-pentane | 13.4 | 0.00439 |
| benzene | 6.91 | 0.0752 |
| toluene | 11.5 | 0.0760 |

No k or Ka is reported for butanone.

## 8. Model-curve reconstruction

The local reconstruction uses:

- Q = 1.428 m3 h-1, corresponding to 23.8 L min-1;
- approximately 3.0 g catalyst in the irradiated zone;
- 24.45 L mol-1 gas molar-volume basis at 25 C;
- pollutant molecular weights for ppmv-to-mg m-3 conversion;
- the reported k and Ka values.

The reconstructed curves are model outputs for visualisation and reproducibility, not additional experimental points.

## 9. Principal limitations

- Single-component adsorption representation.
- Ideal plug-flow representation rather than a complete residence-time-distribution model.
- No explicit radiation-field attenuation submodel.
- No explicit hydrodynamic or attrition submodel in the web reconstruction.
- Parent-VOC conversion does not prove complete mineralisation.
- Butanone has only one successful photocatalytic run and no kinetic fit.