The Preparation And Evaluation Of Coconut Shell-based Carbon Molecular Sieves For Air Separation

Carbon molecular sieves have effective micropore diameters ranging from about 0.4 to 0.9nm, which is comparable with the molecular diameter of the adsorbates to be separated and can be widely used in gas separation processes on a kinetic basis. The most important large-scale application is for air separation. Currently, high-molecular polymer and coal material with low inorganic compounds can be used as raw materials for CMS preparation with excellent air separation performance. However, use of shell from plants as a cheap and abundant carbon source for generation of CMS is still problematic because the technology of adjusting pore structure is not mature, such as poor reproducibility, unstable performance and so on. Therefore, the ability to control the size of the pore openings in the carbon precursor and CMS, to tune the specifications to tenths of an angstrom in the case of air separation, to enhance stability, is still a major challenge in preparing CMS adsorbents.Based on this, the purpose of current paper is developing works around bottlenecks faced by the large-scale preparation of coconut shell-based CMS in the aspect of science and technology. Establishing an evaluation method is to feedback the information of the relative pore mouth of CMSs, then conditions of adjusting pore structure can be controlled to produce CMSs to preserve the high adsorption capacity of the carbon adsorbent, while selectivity would be introduced.Two-column pressure swing adsorption unit is adopted as evaluation device. Two commercial CMSs, Japan CMS and Germany CMS, are used as adsorbents to choose conditions of evaluating the pore mouth sizes of CMSs based on the recovery of N2. Under the conditions of the selected evaluation, it is discovered that N2 concentration and recovery of product gas, the highest concentration of O2 in effluent, the time of the highest O2 concentration in effluent, the amount of effluent is different due to different CMSs, therefore, the relationships between the pore mouth size and characteristics of product gas and efflent such as N2 concentration and recovery of product gas, the highest concentration of O2 in effluent, the time of the highest O2 concentration in effluent, the amount of effluent are discussed.An effort has been made to develop a suitable CMS for oxygen removal from air. The CMS are prepared through char from pyrolysis of coconut shell are mixed with thermosetting phenolic resin and adjusted through the measures of carbonization, physical activation and chemical vapor despostion based on information of pore structure. The results show that with optimum operation conditions of final carbonization temperature 820 ℃,carbonization heating-rate 10℃/min, carbonization times 30 min, activation flow rate 500 ml/min, activation time 90 min, as-prepared activated carbons have the narrowest pore size and the most probable micropore size is 0.5 nm. Then the activation carbon is used as the carbon precursor to preparate CMS by two-step chemical vapor despostion. It turns out that N2 concentration in the product can increase by 2.2% compared with one-step chemical vapor despostion and two-step chemical vapor despostion enhances the performance of separating air. The relation between air separation and the adjusting conditions are attained which can guide the future experiments for prepared CMSs.