Dipl.-Ing. Yanyan Zhao
Diplomprüfung: 09/2008
Diplomprüfung: 09/2008
DyStar Nanjing produces dyestuff for the textile industry, hazardous gases are produced during the production process. The main inlet pollutants are Methanol, Chlormethane and Chlorbenzene. To avoid pollution, the Thermal Oxidizer process (TOx- System) has been put into place to decompose the pollutants into CO2 and H2O. The main process steps are Thermal Oxidation, Cooling/Quench and Scrubbing/Scrubber (see diagram).
From the production, the waste gas is let into the air pre-heater and is heated up to a temperature of 500 °C to reduce the temperature differences between inlet and reaction temperature in the combustion chamber to save energy. In the combustion chamber the waste gas will be oxidized at a temperature of 800-950 °C.
The hot exhaust gas will then pass through the pre-heater to be cooled down to a temperature of 530 °C before entering the quench and scrubber system.
Coming out of the Thermal Oxidizer, the hot exhaust gas will then pass the quench. The quench is installed in vertical arrangement between TOx- System and scrubber inlet with the material of graphit. The hot exhaust air comes into contact with the washing liquid from the scrubbers to cool down the exhaust stream from 530 °C to a Fiberglass Reinforced Plastic harmless temperature in the quench before entering the scrubber inlet. Then the exhaust passes through the wet scrubber and comes into contact with washing liquid and pollutants are removed.
Before the discharge, the treated exhaust passes through a demister, and the solution is collected to the collection tank.
The TOx-plant of DyStar Company came to a deflagration at 4.30 pm on 20th June, 2007. This incidence requires heavy engineering investigation in order to determine the nature and cause of the deflagration. Based on the previously available data and information, such as pictures taken showing the damage pattern of the plant after deflagration, it suggested that the deflagration started at the pre-heater for waste gas within the plant.
The description and evaluation will be carried out, why the TOx-plant of DyStar came to an explosion and what measures could be implemented to that ensure, the plant in the future could be operated safely.
After analyzing the previously available data and information, concerns are raised in particular with the following issues:
(a) Inflammation of explosive waste gas mixtures on the hot surface of the waste gas pre-heater.
(b) Input of combustible condensate in the air pre-heater with subsequent evaporation and formation of explosive gas within the air pre-heater followed by inflammation on the hot surface of the air pre-heater pipe.
(c) Dust explosion on the hot surface of the pre-heater
In order to prevent future incidences, using known data and information and based on the requirements of Ex-Zone 1 for the waste gas, the following measures are proposed:
(a) Installation of an effective condensate trap or pre-scrubber
(b) Installation of flow monitoring
(c) Implement pressure surge protection and ignition source free waste gas fan in the pipeline leading to the burner of TOx-Plant
(d) Parallel installation of two effective flame arresters that are designed and tested to comply with the requirements of flashbacklocks.
(e) Using ignition source free waste gas fans in the waste gas pipeline leading to the chimney such as, in this case, installation using suitable design and material with flashbacklocks in the pipeline
Several improvements, including redesign, reconstruction and new maintenance methodologies at critical areas, have been suggested, some of which have already been implemented in the new plant construction, however, more are yet to be implemented and still require further discussions and evaluations.