Journal of Materials Processing Technology157–158(2004)
290–296
Synthesis of carbon molecular sieves by benzene pyrolysis
over microporous carbon materials
E.David a,∗,A.Talaie b,V.Stanciu a,A.C.Nicolae a
a National Institute of Cryogenics and Isotope Technologies,P.O.Box10,Rm.Valcea,Code1000,Romania
b Department of Chemical Engineering,University of Sydney,Sydney,NSW2006,Australia
Abstract
In the prent work,an effort has been made to develop suitable process conditions for synthesis of carbon molecular sieves(CMS)from the locally available bituminous and walnut shell char(CH,CW).The process involves modifying a carbon support having a majority of micropores with an effective pore size of about4–25˚A.The sieves are contacted with different concentration of a volatile carbon-containing organic compound.The char was crushed,milled and agglomerated with starch and pitch as the binders,and then carbonized in nitrogen atmosphere at650–750◦C for about60min.
In order to narrow down the pore mouth size,benzene was cracked at650–800◦C,for10–60min to faci
litate the deposition of coke on the carbon support.All the products were characterized by analysis of pore size distribution,volume of pores,surface area and adsorption data.The best carbon molecular sieve was obtained for feed benzene entrance in N2of3.5×10−4g/ml,cracking time of30min and pyrolysis temperature of800◦C.The carbon molecular sieves are suitable for gas parations.Their performance was judged by the kinetic paration of Ar–O2mixture.Besides their application to gas paration,the CMS materials are receiving incread attention as catalysts.
©2004Published by Elvier B.V.
Keywords:Carbon molecular sieve;Coal;Benzene;Ar/O2paration
1.Introduction
The u of carbon molecular sieves(CMS)in gas p-aration process is known since decades[1,2].At prent there is noted a rising interest for the development of indus-trial gas-paration process,efficient energetically.The de-velopment of gas-paration technologies is almost entirely bound on the development of specific zeolites and carbon molecular sieves.In the last few years,a great number of p-aration process connected on CMS were marketed[3,4].
受人诟病
The active carbon are products resulted from the process-ing of charcoal solid remains obtained through the carboniza-tion of vegetal products(waste wooden materials,coconut shells,walnut shells,fruit kernels),mineral coals(from an-thracite),waste mineral oils and so on.The active carbons have a great distribution of their pores(2–20,000˚A)and as a result,they practically do not posss a lectivity in the ad-sorption of various gas molecules depending on their sizes,as ∗Corresponding author.it is performed in the ca of some zeolites or carbon molecu-lar sieves.CMS is also applied in systems with more compo-nents which differ very little in their molecular sizes.Thus, the paration of oxygen from nitrogen or oxygen from argon may be performed by kinetic adsorption on carbon molecular sieves.In the view of obtaining the paration,the adsorbent must have the pore size,next to the molecular diameter of the majority gas in the mixture(nitrogen or argon).This allows the fast adsorption of the smaller component and the slower diffusion of the component with greater molecular size,re-sulting thus high kinetic lectivity.
The ability to control in a CMS the exact size of the pores to tenths of an angstrom,like in the ca of air paration or oxygen from argon paration,is a major challenge in prepar-ing CMS adsorbents.While the adsorbent is a key part of the performances in entire process,the improved CMS adsor-bents are necessary to reduce the paration costs through the PSA system.
金文
CMS have a lot of advantages given to the zeolite molecu-lar sieves[5,6],like its high hydrophobicity,its resistance
0924-0136/$–e front matter©2004Published by Elvier B.V. doi:10.1016/j.jmatprotec.2004.09.046
E.David et al./Journal of Materials Processing Technology157–158(2004)290–296291
as in alkaline as in acid medium,and its structural sta-bility at high temperatures.A review of different rearch teams involved in CMS synthesis was reported by Junt-gen[7].The paration of gas mixtures using PSA tech-nique are esntially bad on the differences in the adsorp-tion equilibrium of each or on the differences in the ki-netic adsorption of individual components on an given ad-sorbent[8,9].Regarding the zeolites,many gas parations are bad on the equilibrium adsorption,while at the CMS, the paration is bad either on equilibrium adsorption or on kinetic adsorption.In the equilibrium paration,the ad-sorption forces are responsible for the adsorption,while the kinetic paration is given by the difference of diffusion speed of various gaous molecules in the micropores in-side.
The effects of the kinetic paration,bad on the differ-ences of molecular sizes,may be obtained b
y reducing the pore size.The narrowing of the pore size may lead to the rising of the diffusion speed for smaller molecules compara-tively with tho greater having as result a sharp paration of the gas,bad on the molecule size.The decrea of the pore size may be performed by lading down additional carbon at the pore entrance and in its inner.
CMS can be obtained from various carbonaceous sources by special processing and treatments or from active coals by various treatments,which either narrow the pores or changes the pore’s size distribution in order to produce a ma-terial with a bimodal pore distribution,having in prevalence smaller pores than6˚A[10–12].The problem of obtaining CMS was not a simple one,becau by activation the car-bonaceous materials led to the simultaneous obtaining of a framework of pores with different sizes.The micropores hav-ing about3˚A may be obtained through the pyrolysis of an organic substance in the pores of a carbonaceous material [13].
In varied trials,it was studied the effect of coke deposi-tion through the cracking of hydrocarbon in activated CMS samples[6].The average diameter of the pores can be ad-justed by the intensive treatment on the cracking process. Suitable carbonaceous substances that can be ud in the treatment include benzene,ethylene,ethane,hexane,cyclo-hexane,methanol and so on[7].Upon pyrolysis,carbona-ceous deposits are formed that modify the pore structure of the coal-derived coke,
thus improving its parating ability for O2of N2or Ar.Pitch,bitumen,tar oil,or gaous coking materials are ud as binders to prepare extrudates.
The patents to Kuraray Chemical Co.[7,11,13]described veral process for narrowing the micropores of activated carbon by precipitating soot in micropores and described a method to provide improved lectivity for parating nitro-gen from air.The method involved using coconut shell char-coal and coal tar binder,acid washing,adding coal tar,and heating from950to1000◦C for10–60min.The coal tar penetrates the surface of the active carbon and decompos to grow a carbon crystallite on the inner surface of the mi-cropore.
There are numerous other multi-step synthesis patents that describe pyrolysis of carbonaceous materials onto an acti-vated char[6,8].All the process involve the formation of a gate-keeping layer onto the pores of the carbon via the deposition of a pyrolyzable carbonaceous material.The gate-keeping layer is a region near the pore opening that is narrowed sufficiently to allow O2and N2or O2and Ar to transver,and through which O2pass significantly faster (10–50times)than N2or Ar.
In our numerous attempts to prepare high lective ma-terials for oxygen,the difficult part was the ap
plication of the receipts at carbon layers insufficient defined carbon supports.The carbon deposition procedure is very much de-pendent on both the chemical and physical properties of the support.The carbon supports need to be clearly defined in order to successfully apply the carbon deposit that provide a kinetic barrier and imparts O2lectivity.In order to obtain carbon supports,it is necessary the u of an initial material with absolute uniform properties.For this reason,the raw ma-terials require special treatments before they are transformed in molecular sieves.
The major difficulties met at the processing of CMS are connected on the constant control and maintain of certain working parameters as:temperature of the calcination oven, oven enclosure atmosphere,depositing time in the pores of the carbon which has resulted from carbonaceous substances cracking,the prence or abnce of pores in the initial car-bonic material.The quality control of CMS is performed by the modification of one or of all above-mentioned parameters, obtaining thus various adsorbent materials.
In the prent paper,the roumanian bituminous coal burned(33%ash content)and the charcoal obtained from walnut shells were chon as starting carbon materials for the CMS synthesis.After the preliminary treatments and the charring,benzene was cracked to deposit carbon in the view of control the pores’size.The resulted CMS sam-ples were characterized by measuring the dynamic ad
sorp-tion capacity for oxygen,using a argon–oxygen mixture. It was utilized an adsorption process at variable pressure (PSA).The gasflow rate through the CMS bed has deter-mined the residual oxygen concentration in the produced ar-gon.
2.Experimental
2.1.Materials
The bituminous coal was procured from the“Petrila-mine”,county Hunedoara,Roumania(CH)and the coal which was obtained by carbonization,in earth atmosphere of the walnut shells(CW),were ud as carbon precursors. The binders ud for agglomeration were pitch and starch, which were supplied by the coke works from Petrosani,Rou-mania,and“Reactivul–Bucharest”,Roumania,respectively. All gas utilized were of analytical reagent grade.
292 E.David et al./Journal of Materials Processing Technology 157–158(2004)
290–296
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Fig.1.Procedure for the carbon molecular sieves synthesis.
2.2.Pretreatment of raw materials
The experimental procedure ud for the processing of CMS is shown in Fig.1.The bituminous coal was first crushed (10–30mm),and then dried at a temperature of 130–150◦C for 4h.In this time was removed the water content from the coal.The material was then crushed to particles having the sizes of 0.5–2mm.The sorted mass was subjected to oxida-tion in a fluidized bed with an oxidized mixture of 12.5vol.%O 2in N 2,at a temperature of 250–300◦C for 4–5h.The pur-po of the oxidation was that of removing the hydrogen content,nitrogen,sulphur compounds,and so one,which are prent in the mass of the coal.After the oxidation,it was obtained a carbon support with uniform properties.The car-bon mass was grinded and sorted to particles having the size under 0.12mm.The cond raw material was obtained from walnut shells by their carbonization at 400◦C in a nitrogen atmosphere and it was subjected to the same pretreatment,following the procedure shown in Fig.1.二胡经典曲目
The coal powders obtained from bituminous coal and from walnut shell charcoal are mixed with pitch (dissolved in ben-
Table 1
Conditions for coke deposition by benzene cracking,coke mass deposited,volume of pores and oxygen capacity Sample No.
Cracking Nitrogen flow rate (ml/s)Benzene entrainment (g/ml of N 2)Weight
deposited (mg)Total volume of pores (cm 3/g)O 2capacity at 25◦C (cm 3/g)Temp.(◦C)
Time (min)Coal
––––
–0.532–CMS-H-165010 1.38 1.2×10−44.000.396 4.36CMS-H-270020 4.16 2.56.000.407 5.20CMS-H-380030 5.55 3.511.000.414 5.45CMS-H-480040 6.94 4.55.060.402 4.12CMS-H-5800508.33 5.21.20.385 3.78Walnut shell ––––
–0.378–CMS-H-165010 1.38 1.2×10−44.000.495 5.25CMS-H-270020 4.16 2.57.420.512 6.31CMS-H-380030 5.55 3.514.830.532 6.43CMS-H-480040 6.94 4.56.410.501 5.33CMS-H-5
800
50
8.33
5.2
1.54
0.385
4.88
zene at 70◦C)and starch (dissolved in water at 65◦C).The mixing ratio of the coal powder with pitch and starch was of 20:6:1part weight.The homogeneous paste was extruded in a pneumatic press under the form of pellets with a di-ameter between 4and 5mm and a length of 6–8mm.After drying at 120–130◦C for 4h,the pellets were carbonized in a nitrogen atmosphere,in the reactor (350mm length and 50mm i.d.)that was equipped with a preheater.The tem-perature control was performed by a temperature indicator connected to a precalibrated chromel–alumel thermocouple.The gas flow rate was measured and controlled with the help of precalibrated manometers and regulators.Till 700◦C,the coal mass was heated in steps with 3◦C/min.At tempera-ture of 700◦C the material was carbonized for 30min,af-ter which the heating continued to 750◦C with a rising of 1◦C/min.
2.3.Benzene cracking
The experimental procedure for the CMS synthesis by hy-drocarbon cracking on carbonic mass is reprented by the flow shown in Fig.1.
After finishing the charring stage,performed in a nitrogen atmosphere,coke was deposited on support by controlled benzene cracking under the conditions shown in Table 1.The deposited carbon mass was monitorized by quantitative mea-surements.Pore size distributions were measured by the molecular probe method.Adsorption of various organic va-pors by the carbon sieves over a 72-h period was mea-sured on a sartorius balance.The samples were expod to various organic vapors (p /p 0=0.5)at 25◦C.The follow-ing liquids were chon,becau they had sufficient vapor pressure at 25◦C over the desired range of pore sizes and
with the following densities:NH 3(3.65˚A,
0.904g/cm 3);CH 3OH (4.4˚A,
0.791g/cm 3);CHCl 3(4.6˚A,1.477g/cm 3);n -C 6H 14(4.9˚A,
0.66g/cm 3);CCl 4(6.0˚A,1.594g/cm 3);C 6H 10(6.1˚A,
0.78g/cm 3);C 6H 6(6.8˚A,0.876g/cm 3).The specific surface area and the total volume of pores were deter-mined using mercury porosimetry method,with a porosime-ter of 4000atm Carlo Erba.
E.David et al./Journal of Materials Processing Technology157–158(2004)290–296293 Table2
Parameters of pore structure for the samples CH,CW,CMS-H-3and CMS-
W-3
Sample Carbon support Benzene cracked samples a
CH CW CMS-H-3CMS-W-3
Total volume of
pores(cm3/g)
0.350.370.410.47
0–25˚A0.260.310.360.41
25–200˚A0.030.040.020.03
200–20,000˚A0.060.020.030.03
Specific surface
S BET(m2/g)
国土空间规划编制197210262291
a Cracking temperature800◦C;C6H6entrainment=3.5×10−4g/ml of
N2;nitrogenflow rate=5.55ml/s,cracking time=30min.
2.4.Quantitative measurements
The quantitative measurements were performed using the
volumetric procedure.The gas volume hold back by the sam-
ple in a given interval of time,at a temperature of25◦C and an
initial pressure of8×10−2Torr,was determined from the dif-
ference between the initial concentration of O2in argon and
thefinal concentration of O2in produced argon.There were
plotted the saturation curves with oxygen C O
2(%)=f(time)
for various pressures.It was ud a laboratory PSA system with a single adsorber.
The speed of the gasflow was maintained constantly through CMS bed.The mixture ud5.64vol.%O2in Ar at a working temperature of25◦C.The initial mixture of O2 in Ar and the withdrawn samples were analyd by GC,us-ing a Hewlett Packard Analyr with a thermal conductibility detector.It was determined the dynamic adsorption capacity for oxygen in the pressure range4–10bar.
3.Results and discussion
3.1.CMS characterization
As mentioned in the earlier ctions,active carbon samples were produced by heating suitably prepared coal extrudes in nitrogen atmosphere at700◦C.During this process,the dis-appearing of organic compounds take place which are prent in coal deposits and in the content of the coal obtained from walnut shells.It takes place the rising of the porous carbon mass,which contains a large distribution of pores size.
Table2prents the parameters of pores structure for car-bon samples(CH)and(CW)as much as for CMS-H-3and CMS-W-3obtained after cracking benzene.
In order to prepare the adsorbents having narrower pores in a near-uniform pore size distribution,a hydrocarbon crack-ing technique was followed to reduce the pore mouth opening by coke deposition.The reduction of pore diameter can be accomplished by carbon deposition in the interior of microp-ores.The deposition of carbon in the interior has to be care-fully controlled so that gas diffusion is not limited or
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capacity Fig.2.Deposition of carbon onto/into a pore:(a)uncoated micropore of ∼6–8˚A;(b)deposition of carbon by pyrolysis of benzene into the throat of the pore;(c)deposition of carbon by pyrolysis of benzene into some of the throat of(b),but mainly at the pore mouth.
diminished significantly.Using this concept,we attempted to reduce the pore diameter of carbon support before attempting to neck down the entrance to impart O2kinetic lectivity.To accomplish this we ud the hydrocarbon pyrolysis.
Deposition occurs in the interior of the pore if the control of the cracking is not rigorously controlled(Fig.2b)or con-trolled carbon deposition occurs at the pore entrance(Fig.2c). The treatment with benzene provided the necessary lec-tivity by preferential deposition at the pore mouth,like in Fig.2c.The results are summarized in Table1.By deposit-ing the carbon,resulted from the hydrocarbon cracking,on the wall or mouth of the pores it was reduced their dimension. It is probable that the pores with larger openings could have more carbon molecules resulted from the hydrocarbon crack-ing than the narrow ones,due to a better accessibility of such molecules,leading to more coking in the wider pores.The aim of coke deposition is to reduce the pore opening to the re-quired molecular range,in which ca the kinetic paration with respect to desired molecules is predominant.However, the amount of coke deposition also depends on other factors, like the nature
of the hydrocarbon,their shape and size,de-gree of unsaturation,temperature of cracking,concentration of hydrocarbon in stream,the time of cracking and so on.
A typical micropore size distribution for CH,CW,ud as carbonic supports and CMS-H-3,CMS-W-3ud for Ar–O2 paration is shown in Fig.3.CMS-H-3has a micropore profile similar to of CMS-W-3,but with smaller capacity through the range shown in Fig.3.女生动漫
3.2.Coke deposition by benzene cracking
For reducing of the carbon pore size,benzene was ud as the cracking agent for coke deposition.The nitrogen flow rate was maintained at constant values in the range 1.38–8.33ml/s,also it was maintained constant the ben-zene content in the combined nitrogenflow(in the range 1.2–5.2×10−4g/ml of nitrogen)and the cracking time of hydrocarbon was modified from10to60min.After time of hydrocarbon cracking,through the reactor was pasd for 10min a nitrogenflow to remove the uncracked benzene and the excessive coke deposit.
294 E.David et al./Journal of Materials Processing Technology 157–158(2004)
290–296
Fig.3.Microporosity of CH,CW and CMS adsorbents.
The estimation was performed by utilizing the samples from the kinetic paration of the Ar–O 2mixture.The ki-netic adsorption curves,realized for the samples cracked with benzene,are shown in Figs.4and 5.
From Figs.4and 5it may be obrved that owing the ri of the cracking time there were obtained changes regarding the amount of adsorbed oxygen.The maximum value was obtained for a cracking time of 30min and a benzene content in the combined flow of 3.5×10−4g/ml of N 2.The prolon-gation of the cracking time more than 30min led in fact to a reducing of the oxygen adsorption capacity.The perfor-mances of the cracked samples with benzene may be en in Table 1.The reducing in value of the oxygen quantity for cracked samples with benzene a time greater than 30min may be due the phenomena of excessive coke formation and prob-ably to excessive filling of the pores.The benzene molecules may penetrate deep inside the pores,resulting in coke de-position deep into the pore walls (e Fig.2)rather than
at
Fig.4.Adsorption of O 2on benzene cracked samples (CMS-H benzene en-trainment =3.5×10−4g/ml of N 2,nitrogen flow rate =5ml/s;initial pres-sure =8×10−2
Torr).
Fig.5.Adsorption of O 2on benzene cracked samples (CMS-W benzene entrainment =3.5×10−4g/ml of N 2,nitrogen flow rate =5ml/s;initial pres-sure =8×10−2Torr).会计刺客2
pore months.This type of coking effect results in the reduc-tion of adsorption capacity.It is interesting to note that the CMS-W prepared samples by benzene cracking into precur-sor coal proceeded from the carbonization of walnut shells,have given better results in oxygen adsorption compared with the CMS-H samples produced under similar conditions,but having as precursor bituminous coal (e Table 1).The bet-ter behavior of CMS-W samples may be explained by the fact that this material has a greater volume of micropores with the diameter near oxygen molecule size.The sorbents have prented a more uniform distribution of the microp-ores,which was reflected in the value decreasing of residual oxygen in produced argon by kinetic paration of Ar–O mix-ture.
The carbon support with a more uniform structure was obtained from walnut shell charcoal.The sharing process of walnut shells promotes the creating of pores with sizes and sharps that allow easi
ly the access benzene molecules.In the ca of the bituminous coal,the prence of inherent func-tional groups can hinder the uniform stratification,causing apparent structural defects.The ash content of the bituminous coal is much higher than that of the walnut shell charcoal,and this may be responsible for the difference in coke deposition on the CMS products.
In principle,the hydrocarbon that supplies the carbon for narrowing the pore diameter of the support can be any volatile carbon-containing organic molecule,including hydrocarbons and compounds with heteroatoms such as oxygen,nitrogen,sulfur,silicon and the like,provided that the compound can decompo cleanly to carbon without forming other pore-plugging materials.It is important,however,for the carbon-containing compound under pyrolysis conditions to gave an effective molecular dimension smaller than the majority of the pore openings in the untreated carbon support,but large
