Activated nanocarbon production using physical activation by water steam from agricultural wastes -

Activated nanocarbon production using physical activation by water steam from agricultural wastes

The optimum production conditions by a physical activation method, influence of temperature (850, 900, 950 and 1000 °C), activation residence time (30, 60 and 90 min), and mill rotation (200, 300 and 400 rpm) were investigated using three different raw materials including walnut, almond and pistachio shells. To prepare activated nano-carbon, all the samples were heated up to the final activation temperature under a continuous stream flow of 130 cm³/min and at a heating rate of 3 °C/min and were held at the different activation temperatures for 30, 60 and 90 minutes. BET surface area of the obtained activated carbons was measured from nitrogen adsorption data in the relative pressure range between 0 to 1. Activated nano-carbon standard indexes were evaluated according to the ASTM standard and the samples were compared. First, the cellulose raw material was heated in the carbonization furnace at 600 °C and then activated in the advanced activation furnace at a temperature between 850 to 1000 °C for 30, 60 and 90 minutes with water vapor. Ash percentage, iodine content, moisture content, specific area, elemental analysis, and FESEM were used for product characterization.

Materials

The raw materials used in this study were walnut, almond and pistachio shells. In the first stage to make activated carbon, 300 g of Iranian pistachio, almond and walnut shells were prepared. The high carbon content and lower ash content of these three materials indicate their high potential for producing a high-quality carbon.

Experiments firstly, to remove impurities and dust, the raw material was washed with deionized water and then dried in an oven at 80 °C. Subsequently, the raw materials crushed by a vibrating mill for 30 minutes, and they are classified by a mesh 20 as regular granules. The prepared granules are washed twice with deionized water and then placed inside the oven at temperature 70 °C for 6 h to completely dry. At this stage, 200 g of each sample was placed in special crucibles, and crucibles were placed into the atmospheric furnace in order to carry out the process of carbonization (pyrolysis).

In order to carry out the carbonization process, the furnace condition was adjusted under nitrogen atmosphere with the specified temperature cycles. The samples should be heated up to 600 °C and held at this temperature for 1 hour. Finally, the samples cooled down to ambient temperature with a temperature decay rate of 10 °C/min. In all of the above operations, pure nitrogen gas supplied at the rate of 150 cm³/min in the furnace for inert atmospheric condition.

 The products of the carbonization stage have low adsorption capacity due to low temperature carbonization and the presence of bitumen in the pores between the crystals and the surfaces. In the carbonization (or pyrolysis) process, non-carbon components such as hydrogen, oxygen, and volatiles are released from raw materials in the form of gas, and free carbon forms regular groups of graphite crystals. The carbon pore structure is formed at a temperature of about 600 °C. Most of these pores are blocked by the released bitumen during the pyrolysis process. For this reason, the activation stage must be performed in order to open these pores and be used as an adsorbent.

A high heating speed results in a very rapid decomposition, resulting in the production of a solid with a developed network of medium and large pores with low density and abrasion. The coal produced in the previous stage, after cooling and reaching ambient temperature, was prepared for the activation step. In this step, the conditions for the production were considered at temperature of 850, 900, 950 and 1000 °C, the temperature rise rate was 3 °C /min and the steam flow rate was 130 cm³/min. In the activation process of carbonized materials at temperatures of 850 to 1000 °C with the presence of oxidizing agents of water vapor.

The coal production system is similar to the activation system, with the only difference being that the heater and the oil bath do not have and the nitrogen gas from the cylinder. In each test, 120 grams of coal produced from each raw material is placed inside the crucible. After completion of the activation and cooling the samples, the samples were washed with very dilute hydrochloric acid (0.01 molar) and deionized water (DW) and dried completely in oven at 70 °C for 2 hours. In this step, activated carbon experiments at 77 K. This experiment was carried out by ASAP 2000 equipment (Micrometrics Co.).

Determination of Iodine Number has been performed according to ASTM-D4607 standard. Also, for more accurate examination of the structure of the obtained products and raw material samples, the images were taken by the scanning electron microscope (SEM-360) and electron microscopy FESEM (XL30).

Results and conclusion

The results of the analysis showed that by using the water vapor physical activation method and optimizing the parameters of this process including time and rotation of the mill up to 10 min and 400 rpm, resulted in a significant increase in specific surface area, cavity volume and the iodine number of the final product.

Using physical activation with water vapor and raw materials used in this study, activated carbon granules with high specific surface area was obtained. The highest specific surface area of activated carbon was obtained for pistachio shell during the 30 min activation time and 60 min for almond and walnut shell. The highest specific surface area of activated carbon was obtained after activation and before high-energy milling, 910 m²/g, which belongs to the almond shell with a 60 min residence time. Walnut shell had the best yield and lowest burn-off and almond shell had the lowest yield and highest burn-off. The vibrating mill has not been able to produce nano activated carbon particles and finally, with a high residence time up to the micro scale (above 100 nm) it can mill these particles. High energy mills (planetary mills) are capable of producing activated nano-carbon. As the mill round increases, the specific surface area of the powder increases and smaller particles are produced. The highest specific area after planetary milling was for almond and walnut shell, respectively at 400 rpm (1261 and 1235 m²/g, respectively), but the optimum milling speed was 300 rpm. Adjusting the dimensions of the holes and how they are distributed plays a major role in the process of activated nano-carbon production because these features affect the enduse of the product. Activated carbon with fine holes smaller than 2 nm is suitable for adsorption from the gas phase, whereas a sample with high aggregation of medium holes with dimensions ranging from 2 to 50 nm is more suitable for adsorption in liquid phase. Almond shell as a cellulosic material with a cellular and porous structure has shown a very good potential as a raw material for the production of highly specific surface activated nano-carbons. Changing the type of milling of samples, results in a significant increase in the free surface area and volume of the cavities. The iodine content of the products has also increased significantly. Given the near-surface specificity of the two samples, the particle size of the two samples was predicted to be similar, and the FESEM results confirm this prediction. Finally, using the assumptions and methods adopted in this study, one can achieve nano-sized activated carbon powder by using agricultural wastes such as pistachio, almond and walnut shells, by physical activation with water vapor and high-energy mechanical milling.

Reference:

Nazem, M.A., Zare, M.H. and Shirazian, S., 2020. Preparation and optimization of activated nano-carbon production using physical activation by water steam from agricultural wastes. RSC advances, 10(3): 1463-1475.