Thermal and Mechanical Analysis of Rotary Kiln of Activated Carbon Producing Reactor

Activated carbon has a wide range of applications as a porous material in the liquid or gas phase adsorption process. The physical process of activated carbon production is divided into two stages thermal decomposition and activation. In this study, only the activation stage has been studied because it is very important in the properties of activated carbon being produced.The production of activated carbon from horticultural waste not only leads to cheap production and supply of many industrial and environmental necessities but also reduces the amount of the produced solid waste. Iran produces about 94,000 tons of pistachio husk annually, which is a good raw material for the production of activated carbon. The profitability index of activated carbon production in Iran is equal to 3.63, which in the case of export, the profitability index will be tripled.Studies have shown that temperature, period, and activation gas flow are the key factors affecting burn-off and iodine number during activated carbon production. Among the various activators tested, steam was found to be the most efficient, with the fastest activation time. For pistachio crops, the minimum iodine number required for economic efficiency is 600 mg g-1, while the highest specific surface area according to the BET test is 1062.2 m2 g-1.

A Mannesmann tube made of 10 mm thick steel was used to construct the rotating reactor. To minimize heat loss during operation, the kiln body was insulated with a ceramic blanket capable of withstanding temperatures up to 1400°C. The kiln had a length and diameter of 190 cm and 48 cm, respectively, and operated at a temperature of 600°C, requiring approximately 25 kWh of energy for heating. CATIA V5 R21 software was employed to design the device, while ANSYS R20 software was used for thermal and mechanical analysis. The rotary reactor was identified as a critical component due to the high levels of thermal and mechanical stress it experiences. To address these issues, a thermal and fluid analysis was conducted, followed by a mechanical analysis using the results from the prior step. Subsequently, experimental tests were performed on the actual model, and the results were analyzed using statistical methods, including the T-student test in IBM SPSS software.The central heating unit and its surroundings were modeled using ANSYS CFX to obtain valuable information on fluid velocity, radiant properties, and heat transfer within the kiln and surrounding area at an operating temperature of 650°C. The analysis revealed uniform steam flow velocity between the kiln and the heating unit. To accommodate longitudinal expansion resulting from heat stress, taller rollers were employed to allow freedom of movement in that direction, while the lateral movement was unrestricted. This arrangement allows the reactor length to increase under varying temperatures. The reactor's end was designed with grooves and pressure plates, incorporating abrasion and compression plates made from refractory fibers to effectively seal the device. Furthermore, telescopic movement of the parts compensates for expansion effects.

The operating temperature of the system was gradually increased to reduce thermal stresses in the reactor shell. This led to a maximum increment in a longitudinal increase of 11.75 mm. Results from five sets of experimental tests and five software analyses demonstrated no significant differences between the experimental and analytical results at a significance level of 5%. Based on the thermal contour analysis, the thickness of the insulation layer was determined to be 5 cm. To control the operating temperature of the device, two methods were employed: adjusting the flame length of the burner and using different types of exhaust outlets. These measures effectively reduced thermal stress on the device.

Thermal and mechanical analysis were useful methods for predicting heat distribution, thermal stresses, and potential dimensional changes in the activated carbon reactor. To compensate for possible alterations in the reactor's length and diameter, abrasive plates and friction washers were implemented. Careful control of fuel input to the burner and regulation of exhaust gas flow helped effectively reduce thermal stresses on the device.