In the present work, a series of
MOF-74 (Ni) materials with narrow micropore channels and abundant
unsaturated metal sites was respectively prepared via hydrothermal
(HT), condensation reflux (CE), and microwave-assisted (MW) methods.
The physicochemical properties of synthesized materials were characterized
by powder X-ray diffraction, N2-sorption, field-emission
scanning electron microscopy, Fourier-transform infrared (FTIR), thermogravimetric
(TG)/TG-FTIR, X-ray photoelectron spectroscopy, UV–vis–near
infrared, NH3/CO2-temperature programmed desorption,
and in situ diffuse reflectance infrared Fourier
transform spectroscopy. Their CO2/N2 adsorption
performances were evaluated by isotherm adsorption and dynamic adsorption
experiments. We found that the MW is a rapid and facile protocol for
the synthesis of MOF-74 (Ni) materials with highly efficient CO2 capture capacity. The well-shaped MW-140 adsorbent with superior
CO2 adsorption capacity of 5.22 mmol/g at 25 °C can
be obtained within 60 min by the MW process, almost 6 times higher
than that of the commercial activated carbon (0.89 mmol/g). Results
of dynamic adsorption experiments showed that the MW-140 material
possesses the highest CO2 adsorption capacity of 3.37 mmol/g
under humid conditions (RH = 90%). Importantly, MW-140 has excellent
adsorption stability and recyclability, superior CO2 capture
selectivity (CO2/N2 = 31), and appropriate isosteric
heat in CO2 adsorption (21–38 kJ/mol), making it
a promising and potential material for industrial CO2 capture.
Characterization results demonstrated that the high capture capability
of MOF-74 (Ni) materials can be attributed to the synergistic effect
of abundant narrow micropore channels and rich five-coordinated Ni2+ open metal sites which are beneficial for the trapping of
CO2 molecules.
To reduce the regeneration costs of capturing CO2 with absorbents, a two-dimensional desorption model for two-phase flow has been proposed. The temperature and pressure predicted by the model were found to agree well with the experiment data. A new synergy pinch concept was proposed to determine the interactive synergy impacts on mass and heat transfer. According to the proposed synergy pinch principle, the carbon surplus (SAC) and energy surplus (SAE) were defined to quantify the synergy effects for mass and heat transfer, respectively. The proposed synergy pinch principle is that the lower the values of SAC and SAE, the more effective the CO2 desorption. In addition, the synergy pinch point can be identified at the ideal value of zero SAC and SAE. Based on this analysis, a stripper with fin internals was introduced that could contribute energy savings of about 10–14% by synergizing the fluid flow between mass and heat transfer. Thus, it was verified that the synergy pinch method can be effectively applied in CO2 emissions control.
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