Patent Application: US-201414491015-A

Abstract:
sorbent for reversible warm co 2 capture . the sorbent includes activated carbon impregnated with magnesium oxide , wherein the magnesium oxide constitutes at least 5 % by weight of the sorbent .

Description:
the sorbent material of the invention was synthesized using the incipient wetness impregnation method , which has been widely adopted for preparing heterogeneous catalysts . those of skill in the art will recognize that other methods for depositing mgo on the support may be used including co - precipitation , sol gel method , electrolytic processing , and chemical vapor deposition . an mgo / carbon sorbent can be made in one step using mgcl 2 as a chemical activation reagent for activated carbon production . this process makes the production cost very low , which is a key factor when choosing a sorbent . to achieve the best performance , different magnesium precursors ( nitrate , citrate , acetate , oxalate ) were tested and compared . nitrate was finally chosen as the precursor . several preparation conditions , such as precursor concentration , support materials , calcination temperature and time were also studied to determine the optimal preparation procedure . the finalized impregnation procedure is as follows : 3 . 8 m solution of mgcno 3 ) 2 ( from sigma aldrich ) was added dropwise to preheated activated carbon ( darco ®, from sigma aldrich ), to reach the desired mgo loading ( 15 % by weight in a preferred embodiment ). the as - prepared material was first dried for 12 h in open air , and was further dried in an oven at 100 ° c . for another 12 h . the material was finally calcined in a muffle furnace at 500 ° c . for 3 h under pure nitrogen . it is recognized that other high surface area supports beyond activated carbon may be used in the present invention including metal - organic frameworks ( mof ), zeolites , activated alumina and mesoporous silica . some of these support materials might require modification of the impregnation method such as , for example , to use a nonaqueous solvent rather than water . the formation of the sorbent from the supported magnesium precursor was characterized by thermogravimetric analysis ( tga ) in a thermogravimetric analyzer ( q500 tga , ta instruments ). the typical thermogravimetric profile of the material is shown in fig1 . the material showed three decomposition steps leading to the corresponding sorbent . a first region showed water loss with a broad peak from 350k - 450k . most of the loosely held water in the precursor was lost before the materials reached 463 k . the second region ( 450k - 700 k ) showed an intense endothermic peak which corresponds to decomposition of magnesium nitrate to magnesium oxide . the third region ( 700k - 900k ) has no observable peak , and the small weight change might arise from the weight loss of the carbon support . a temperature of 750k was finally chosen as the calcination temperature , as this would ensure the minimum sintering and complete decomposition of magnesium nitrate . the surface area and pore size distribution of the sorbent were determined by n 2 adsorption - desorption isotherms at 77 k on a micromeritics asap 2020 instrument . the surface areas and average pore diameters ( dp ) were calculated using the brunauer - emmett - teller ( bet ) method . the pore size distributions and pore volumes ( vp ), between 170 and 300 nm , were determined from the desorption branch using the barrett - joyner - halenda ( bjh ) method . see fig2 . as shown in fig3 , both of the materials show characteristic hysteresis loops of the type iv isotherm 14 , which is associated with capillary condensation taking place in mesopores . this type of isotherm is very common in mesoporous industrial adsorbents . analysis of the hysteresis loops indicates that samples synthesized by incipient wetness impregnation did not exhibit any limiting adsorption at high relative pressure , which is characteristic of type h3 hysteresis loops , corresponding to aggregates of plate - like particles giving rise to slit - shaped pores 14 . in general , the porous structure patterns measured by bet were maintained after the impregnation and the subsequent thermal treatment despite the observed decreases in surface area . the morphological properties of the support and sorbent are summarized in table 1 . as expected , heat treatment of the samples reduced their specific surface areas and mesopores volumes . but the average pore diameter does not change significantly . these values ensure that the pore size is not limiting with respect to the subsequent co 2 adsorption studies . the total volume was contributed mostly from mesoporosity volume . the measurements of sorption capacity and cyclic stability of the sorbent were performed in a high pressure microbalance shown in fig4 ( d110 , from thass company , germany ) and a tga analyzer ( q 500 tga , ta instruments ). a buoyancy correction has been considered . the combined isotherm containing reversible and irreversible components was first determined , and the sample was then evacuated to remove only the reversibly adsorbed co 2 . the adsorption isotherm was then measured again , yielding the reversible adsorption only . the irreversible adsorption was determined by the difference between the total isotherms and reversible isotherm at different pressure points . the capacity for reversible and irreversible adsorption at different temperatures are summarized in table 2 . concerning the effect of temperature , it was observed that the capacity of the adsorption decreases as the temperature increases , because high temperature shifts the adsorption reaction equilibrium . the reversible adsorption capacity shows a strong dependence on the partial pressure of co 2 , and the trend can be fitted to a langmuir model with good precision . the irreversible adsorption capacity reaches a relatively high value even at a low pressure , and the pressure increase does not significantly increase the irreversible adsorption capacity . the irreversible adsorption sites get saturated even at lower pressure , suggesting the irreversible adsorption arises from a strong chemical reaction , which is sharply different from the reaction mode contributing to the reversible adsorption . for a practical pressure swing absorption sorbent , we care only about the reversible adsorption , because only reversible components would contribute in a real application . a reversible adsorption isotherm at four different temperatures was measured in the high pressure microbalance , and the measurement has been corrected by considering buoyancy effects . a temperature - dependent langmuir model ( equation 1 ) was adopted to fit the isotherm data . as shown in fig5 , the model captures the shape and temperature - dependent behavior . the heat of adsorption is rigorously derived in the literature 15 - 17 as shown in equation 2 , where n i 0 is the capacity . once a temperature - dependent equilibrium isotherm is established , the heat of adsorption can be theoretically derived from an isotherm model based on equation 3 . the heat of adsorption for this sorbent is shown in fig6 , which is consistent with reported data 18 . the value is very close to the enthalpy of reaction between magnesium hydroxide and co 2 19 which is consistent with the analysis concerning the correlation between reaction mode and reversibility 18 . as stressed above , to be applicable in the pressure swing adsorption process , regenerability is an important property . to study the adsorption reversibility , an 84 - cycle test was performed in a high pressure microbalance . the temperature was maintained at 200 ° c . the sorption step was at 13 atm and the desorption was at 1 atm . it was observed ( fig7 ) that the capacity at the 1st cycle is large , which is attributed to the combination of reversible and irreversible adsorption . the capacity decreases dramatically after the first cycle . roughly 30 % of the capacity cannot be regenerated using pressure change only as discussed above . the sorbent can only be completely regenerated at high temperature ( 723k ). after the first cycle , the sorbent can maintain a capacity of roughly 1 . 4 mmol / g . according to the study 18 , the irreversibility arises from the formation of unidentate carbonates . the decrease of surface area could also contribute to the capacity reduction . the sorbent capacity is lower than that of the commonly reported activated carbon and zeolites . but note that those sorbents are working at much lower temperature , and at low temperature , multilayer adsorption contributes to the high capacity . these multilayer sorbents do not work in our desired temperature range . on the other hand , calcium oxide and lithium based materials have a very large capacity at high temperature , but in those cases , the sorption is based on bulk reaction . the drawback of that sorption mode is the extremely slow kinetics associated with bulk reaction and species diffusion in a solid . the novel sorbent disclosed herein apparently captures co 2 based on purely a surface reaction . one can treat it as a pseudo - single layer adsorption . considering the surface area of the sorbent , the capacity data is reasonable . adsorption rate is another important criterion in selecting a sorbent . to be applicable for pressure swing adsorption , the sorption rate needs to be fast to reduce the overall cycle time . fig8 indicates that the weight increases rapidly in the first 2 minutes . the differential weight change indicates significant sorption in less than 1 min . concerning the effect of temperature , we note that the sorption capacity decreases as temperature increases . but the sorption rate is slightly faster at higher temperature which is typical behavior for a chemical reaction . fig9 shows sorption rate at different temperatures . compared with the sorption rate of commercially - available low temperature psa sorbents , like activated carbon and zeolites , the sorption rate of our new sorbent is slower . that is because these sorbents capture co 2 purely based on weak physical interaction . in those cases , the limiting step is mass transfer . however , in our case , the sorption mechanism is based on a surface chemical reaction . it is reasonable to observe a relatively slower sorption rate . it should be noted that it is a common problem for high temperature sorbents to have slow sorption kinetics , because the sorption is based on chemical reaction , surface or bulk , rather than a weak physical interaction with lower energy barrier . sorbents like calcium oxide and lithium - based materials capture co 2 based on bulk reaction . the sorption kinetics can only be improved by increasing the working temperature or by doping which improves the bulk phase mass transfer . but the drawback for this sorption mode is bulk reaction normally leads to a change of crystal structure of sorbent . the sorbent needs a higher temperature to be regenerated , which always results in a decreasing working capacity . the material disclosed here captures co 2 based on a surface reaction . what we want to stress here is , a sorbent with high surface area , high surface biding site density which captures co 2 based on a weakly - exothermic surface reaction would reasonably be the better candidate for the psa process in our temperature of interest ( 200 - 300 ° c .). to compare our sorbent with the well - studied htls discussed above , a reversibility and kinetics comparison was performed in a tga at 200 ° c . and 1 atm co 2 . as shown in fig1 , all materials show a large capacity in the first cycle , but the capacity of htls dramatically decrease after the first cycle , indicating most of the surface reaction is irreversible at this temperature . the capacity for htls keeps decreasing in the 10 - cycle tests , while the mgo / c sorbent maintains a stable and larger reversible capacity . the sorption rate for these three samples was also compared . as shown in fig1 , the mgo / c has a much faster sorption rate compared with htls at 200 ° c . these two comparisons indicate mgo / c is a better candidate for warm co 2 capture in the temperature range of interest . mgo / c provides advantages over htls in both regenerable capacity and sorption rate . the superscript numbers refer to the list of references included herewith , the contents of all of which are incorporated herein by reference . the sorbent disclosed herein has been rigorously analyzed in “ analysis of adsorbent - based warm co2 capture technology for integrated gasification combined cycle ( igcc ) power plants ,” ind . eng . chem . res ., the contents of which are incorporated by reference . it is also recognized that a sorbent within the scope of this invention may begin with a magnesium compound other than mgo such as mg ( oh ) 2 or mgco 3 that will convert to mgo during operation thus failing within the scope of the appended claims . it is recognized that modifications and variations of the invention will be apparent to those of ordinary skill in the art , and it is intended that all such modifications and variations be included within the scope of the appended claims . 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