Regenerative Thermal Oxidation of Airborne N, N-Dimethylformamide and Its Associated Nitrogen Oxides Formation Characteristics
Abstract In this study, a two-bed electrically heated regenerative thermal oxidizer (RTO) was used to test the thermal destruction and oxides of nitrogen (NOx) formation characteristics in burning airstreams that contain either N, N-dimethylformamide or dimethylformamide (DMF) mixed with methyl ethyl ketone (MEK). The RTO contained two 0.152 m × 0.14 m × 1 m (L × W times] H) beds, both packed with gravel particles with an average diameter of approximately 0.0111 m and a height of up to 1 m with a void fraction of 0.42 in the packed section. The thermal recovery efficiency (TRE) and the gas pressure drop over the beds were also studied. Experimental results reveal that, with a valve shifting time (t s) of 1.5 min, a superficial gas velocity (U g) of 0.39 m/sec (evaluated at an influent air temperature of around 30 °C) and preset maximum destruction temperatures (T S) of 750–950 °C, no NOx was present in the effluent gas from the RTO when it was loaded with DMF-free air. When only DMF was present in the influent air, the average destruction efficiencies exceeded 96%, and increased with the influent DMF concentration from 300 to 750 mg/N∙m3. The “NOx-N formation/DMF-N destruction” mass ratios were in the range 0.76–1.05, and decreased as the influent DMF concentration increased within the experimental range. When both DMF and MEK were present in the influent gas, the NOx formation ratio was almost the same and the DMF destruction efficiency increased with the influent MEK/DMF ratio from 150/300 to 4500/300 (mg/mg) and in the preset temperature range. The NOx formation ratios were in the range 0.75–0.96. The TRE decreased as U g increased but was invariant with T s. The Ergun equation was found to suffice in the estimation of the pressure drop when the gas flowed over the packing beds.
Heat Transfer Model for Regenerative Beds
Regenerative catalytic oxidizers (RCO) are an economic and effective means of controlling volatile organic compounds with concentrations exceeding 1,200 mg/m3 in a gas stream. However, factors influencing the performance of RCO when treating volatile organic compounds in gas streams have seldom been addressed. Therefore, this study presents a convection-dispersion model with an effective thermal diffusivity αe as a parameter to simulate the performance of RCO. To verify the effectiveness of the proposed model, a pilot-scale RCO was constructed with two 20 × 200 cm (inside diameter × height) regenerative beds. Gravel was used as the thermal regenerative solid material. Experimental results indicated that the model with an αe of 2.0–3.8 × 10−6 m2/s can be used to describe the time variation of solids temperature with the packing height at superficial gas velocities Ug of 0.080–0.382 m/s. Values of αe for the bed are closer to those for the gravel solids (αs = 1.0 × 10−6 m2/s) than for air (αg = 54 × 10−6 m2...
X-rays and photocarcinogenesis in hairless mice
It is well known that excessive X-ray radiation can cause non-melanoma skin cancers. With the increased incidence of sun-related skin cancer there is a need to investigate the combination of sunlight and X-rays. Immunocompetent C3.Cg/TifBomTac mice (n = 298) were divided into 12 groups. Mice were irradiated with 12, 29 or 50 kV X-rays. The mice received a total dose of 45 Gy. They were irradiated with 3 SED simulated solar radiation (SSR) either before or after irradiation with X-rays. The groups irradiated with X-rays alone, 0, 3, 9 and 10 mice (0, 12, 29 and 50 kV, respectively) developed squamous cell carcinoma. In the groups irradiated with SSR after X-rays the development of tumours was significantly faster in the 50 kV group than in the corresponding control group (175 vs. 194 days, p < 0.001). In the groups irradiated with SSR prior to the X-ray radiation the development of tumours was significantly faster in the 29 and the 50 kV groups than in the corresponding control group (175 vs. 202 days, p < 0.001 and 158 vs. 202 days, p < 0.001, respectively). In conclusion, X-ray radiation alone is a weak carcinogen in hairless mice. There is an added carcinogenic effect if X-ray radiation is given on prior sun-exposed skin or if the skin is sun-exposed after X-rays. We still believe that X-ray radiation is a safe and effective therapy for various dermatological diseases but caution should be observed if a patient has severely sun-damaged skin or has a high-risk sun behaviour.