Process optimization of porous carbon preparation using phosphoric acid and sulfur impregnation.

摘要

Adsorption is a well-established unit operation in chemical process industries utilizing variety of adsorbents having varied adsorption characteristics. The present work attempts to utilize date palms pits as a suitable precursor for preparation of porous carbons with phosphoric acid as an activating agent. The experimental methods reported in literature were synthesized and an appropriate method was chosen to simplify the process. The process optimization was performed using the popular Response Surface Methodology (RSM) adopting a Box-Benkhen design, involving three independent optimizations. In the first case, process optimization was performed to maximize the activated carbon (AC) yield and the methylene blue (MB) adsorption capacity, while in the second case the objective was to maximize the yield and BET surface area with the process variables being activation temperature, activation time and impregnation ratio.;The AC samples were characterized using nitrogen adsorption isotherms for the estimation of BET surface area, pore sizes and distribution of pores, Scanning Electron Microscope (SEM), Fourier Transform Infrared (FT-IR), while the adsorption capacity was estimated using the methylene blue (MB) adsorption tests. The optimized experimental conditions for maximizing the yield and MB adsorption capacity was identified to be an activation temperature of 400o°C, Impregnation ratio (IR) of 3 and activation time of 58 min, with the resultant AC yield of 44% and MB adsorption capacity of 345 mg/g. The textural characteristics of the AC reveal the BET surface area to be 725 m2/g, with pore volume of 1.26 cc/g and an average pore diameter of 2.91 nm. The MB adsorption isotherms were experimentally generated and were tested with the popular Langmuir and Freundlich adsorption isotherm models, to identify the model that match with the experimental data. The maximum monolayer adsorption capacity of the MB was estimated to be 455 mg/g, which compares with the highest of MB reported in literature, evidencing the suitability of the AC for adsorption of macro molecular compounds. The kinetics of methylene blue (MB) adsorption was assessed by varying the initial concentration and the adsorption temperature. The kinetic parameters were evaluated applying different kinetic models by minimizing the error between the experimental data and the model prediction. Among the models tested a pseudo second order model and Elovich's model were found to represent the experimental data more aptly.;The optimum conditions for maximizing the yield and BET surface area was identified to be an activation temperature of 500 °C, impregnation ratio (IR) of 2 and activation time of 75 minutes with the resulting yield and BET surface area of 46 % and 838 m2/g. The pore volume and pore diameter of the optimized sample was estimated to be 1.07 cc/g and 1.69 nm respectively. The low activation temperature, activation time with highest of yield renders the above process technically/economically attractive for commercial manufacture.;Sulfur impregnated carbons are being widely used in gas processing industry for removal of mercury. The process of preparation of sulfur impregnated carbons was found to vary widely adopting different experimental techniques and conditions. In order to identify optimum process conditions for maximizing the adsorption of mercury, process optimization was performed with the process variables being impregnation temperature, impregnation time and carbon to sulfur ratio (CSR). The process conditions were optimized for maximizing the response variable of liquid phase mercuric chloride (HgCl2) adsorption. Sulfur impregnation experiments were conducted. Finally the HgCl2 adsorption experiments were run and optimization was done to maximize the adsorptive removal of HgCl 2 from aqueous solution with the response variables being impregnation temperature, impregnation time and sulfur to carbon to ratio (SCR). The optimum conditions being an impregnation temperature of 544 °C, carbon to sulfur ratio 0.53 and impregnation time to be 43 minutes with 85 mg/g HgCl2 adsorbed. Langmuir adsorption isotherm fitted well with the experimental values and the maximum monolayer adsorption capacity found out to be 294 mg/g. The SEM-EDX analysis was done which suggested the uniform distribution of sulfur over the sample. A comparison study was done between different samples prepared in lab and the samples available commercially. The results showed that the sulfur impregnated sample exhibited the highest adsorption capacity.