Preparation of molded biomass carbon from coffee grounds and its CH4/N2 separation performance
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摘要: 本文以咖啡渣为原料,硅酸钠为黏结剂和造孔剂,通过挤条成型技术制备柱状炭前驱体,经高温炭化活化和碱洗除硅,获得高强度柱状多孔炭吸附剂(CGCs),研究其CH4/N2的吸附分离性能。红外光谱分析结果显示9wt%硅酸钠溶液与原料质量比为1.5的样品CGC-1.5含有丰富的含氧官能团。CGCs的比表面积和孔容积随着前驱体中硅酸钠含量的增加而增大,其中CGC-1.5的比表面积为527 m2·g−1,总孔容为0.33 cm3·g−1。氮吸附等温线和CO2吸附等温线分析结果表明CGCs含有丰富的微孔、介孔以及大孔(个别样品),微孔主要集中在0.48 nm左右。在298 K和0.1 MPa条件下CGC-1.5对CH4的平衡吸附量为0.87 mmol·g−1,CH4/N2 (3/7)的IAST分离选择性达到10.3,优于多数生物质基多孔炭固体吸附剂和晶态材料。双组份动态穿透测试结果证实该材料在常压和加压条件均具有优异的CH4/N2动态分离性能,298 K时0.11 MPa和0.5 MPa条件下的动态选择性分别达到10.4和17.9,经10次吸-脱附循环测试,吸附量保持不变。CGC-1.5的机械强度高达123 N·cm−1,具有潜在的工业应用前景。Abstract: Coffee grounds are promising precursors for excellent porous carbon adsorbents. During the preparation of the porous carbons, sodium silicate was used as a binder and pore-forming agent, and extrusion molding technology was used to prepare them in a columnar form. After carbonization, steam activation and silica removal by alkaline washing, high-strength columnar porous carbon adsorbents (CGCs) were obtained. Their CH4/N2 separation performance was studied by multicomponent breakthrough experiments. The Brunauer-Emmett-Teller (BET) surface area of CGC-1.5 (where 1.5 is the mass ratio of a 9 wt% sodium silicate aqueous solution to the coffee grounds) is 527 m2·g−1. Both the N2 and CO2 adsorption isotherms show that the CGCs are rich in micropores and mesopores, with the micropores mainly centered at about 0.48 nm. FT-IR results show that CGC-1.5 has abundant oxygen-containing functional groups. At 298 K and 1 bar, its equilibrium adsorption capacity for CH4 is 0.87 mmol·g−1, and the separation selectivity for a CH4/N2 mixture (3/7, vol/vol) is 10.3, which is better than most biomass-based porous carbon adsorbents and crystalline materials. Dynamic breakthrough tests show that CGC-1.5 has an excellent CH4/N2 separation performance at both high and atmospheric pressures. The dynamic selectivities at 298 K, 1.1 bar and 5 bar are 10.4 and 17.9, respectively. The adsorption capacity is unchanged after 10 adsorption-desorption cycles. The mechanical strength of CGC-1.5 is as high as 123 N·cm−1, which meets the criteria of industrial applications.
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Key words:
- Coffee grounds /
- Extruded /
- Porous carbon /
- CH4/N2 separation
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图 3 除硅前后CGCs样品的(a) 77 K N2吸脱附等温线和 (b) 孔径分布图(DFT方法), CGCs样品的 (c) 273 K CO2吸脱附等温线和(d) 微孔孔径分布图和累计孔容(DFT方法)
Figure 3. (a) N2 adsorption isotherms at 77 K of CGCs before and after removal of silica and (b) pore size distributions (DFT model) from N2 adsorption at 77 K, (c) CO2 adsorption isotherms at 273 K of CGCs,(d) micropore size distributions and cumulative pore volumes(DFT model) from CO2 adsorption at 273 K
图 4 CGCs在(a) 273 K和 (b) 298 K下的CH4和N2静态吸附曲线, (c) CGCs对CH4/N2的IAST吸附选择性(CH4∶N2=3∶7), (d) CGC-1.5与文献报道的部分吸附剂在CH4吸附容量与CH4/N2吸附选择性方面的对比(CH4∶N2=3∶7, 298 K, 0.1 MPa)[5-7, 10, 12, 15, 20, 21, 27, 30]
Figure 4. CH4 and N2 adsorption isotherms of CGCs at (a) 273 K and (b) 298 K, (c) IAST-predicted selectivities of CGCs at 298 K (CH4/N2=3∶7), (d) the comparison between CGC-1.5 and other adsorbents on CH4/N2(3∶7) selectivity and CH4 uptake at 0.1 MPa and 298 K [5-7, 10, 12 , 15, 20, 21, 27, 30]
图 5 CGC-1.5在不同测试条件下CH4、N2混合气动态穿透曲线 (a) 298 K和0.11 MPa, (b) CGC-1.5和CGC-0动态穿透曲线对比图,(c) 298 K和0.5 MPa, (d)水汽条件下动态穿透曲线
Figure 5. Breakthrough curves for CH4/N2 mixture of CGC-1.5 at different test conditions (a) 298 K and 0.11 MPa, (b) comparison of breakthrough curves between CGC-1.5 and CGC-0, (c) 298 K and 0.5 MPa, (d) breakthrough curve tested in humid condition
图 6 在298 K和0.11 MPa条件下CGC-1.5的(a) 吸-脱附循环测试, (b) 10次循环测试CH4动态吸附量, (c) CGCs的机械强度
Figure 6. (a) Cycle test of CH4 adsorption-desorption on CGC-1.5 at 298 K and 0.11 MPa, followed a regeneration by Ar flow at 298 K, (b) CH4 uptakes on CGC-1.5 at 298 K in 10 times adsorption-desorption cycles, (c) mechanical strength of CGCs
表 1 CGCs的孔结构参数
Table 1. Structural parameters of CGCs
Sample SBET
(m2·g−1)Smic
(m2·g−1)Vtotal
(cm3·g−1)Vmic
(cm3·g−1)CGC-0 447 371 0.19 0.14 CGC-1.1 242 177 0.16 0.07 CGC-1.1
(with SiO2)241 193 0.11 0.07 CGC-1.3 335 228 0.23 0.09 CGC-1.5 527 371 0.33 0.15 表 2 CGCs在298 K和0.1 MPa条件对CH4、N2的吸附容量及IAST选择性(CH4∶N2=3∶7)
Table 2. CH4 and N2 adsorption capacities of CGCs at 298 K, 0.1 MPa and the IAST selectivities of CGCs (CH4∶N2=3∶7)
Sample CH4 adsorption capacity (mmol·g−1) N2 adsorption capacity (mmol·g−1) IAST Selectivity CGC-1.1 0.70 0.20 7.8 CGC-1.3 0.73 0.19 6.6 CGC-1.5 0.87 0.20 10.3 表 3 CGCs在298 K下对CH4和N2的Langmuir-Freundlich拟合参数
Table 3. Langmuir-Freundlich fitting parameters of CH4 and N2 on CGCs at 298 K
Sample Qs
(mmol·g−1)K*10−3
(kpa)m R2 N2 CGC-1.1 0.40 8.80 1.06 0.99997 CGC-1.3 0.37 9.84 1.07 0.99996 CGC-1.5 0.37 10.35 1.08 0.99993 CH4 CGC-1.1 1.47 8.57 0.90 0.99995 CGC-1.3 1.50 8.79 1.00 0.99979 CGC-1.5 1.96 7.29 0.89 0.99997 表 4 在298 K, 1.1 bar条件下CGCs的CH4、N2动态吸附数据
Table 4. Dynamic adsorption capacity of CH4 and N2 on CGCs at 298 K and 1.1 bar
Sample CH4 dynamic adsorption capacity
(mmol·g−1)N2 dynamic adsorption capacity
(mmol·g−1)Separation time
(min)SCH4/N2 CGC-1.5 0.25 0.056 15 10.4 CGC-0 0.31 0.507 0 1.4 -
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