Catalysis Today 242 (2015) 184192

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Catalysis Today

j o ur na l ho me page: www.elsev ier .com/ locate /ca t tod

Chemic onFischer ticsproces

M. Iglesia Engler-Bunte anyb DST-NRF Cen y of CaSouth Africa

a r t i c l e i n f o

Article history:Received 10 March 2014Received in revised form 14 May 2014Accepted 16 MAvailable onlin

Keywords:Iron catalystFischerTropsCO2 hydrogenWater vaporSNG

a b s t r a c t

The potential of a new practical application of FischerTropsch synthesis is investigated, the productionof C24 components to increase the heating value of substitute natural gas (SNG), starting from CO2 andH2, produced from renewable electricity. This process route offers the possibility to convert electrical

1. Introdu

The incrprimary enthe developtrical grid. with a highage, distribfor mediumical fuels frcompletely

CorresponE-mail add

http://dx.doi.o0920-5861/ ay 2014e 2 July 2014

chation

energy into chemical energy. The resulting chemical energy carrier can be stored in the natural gas grid,easy to distribute.

An iron-based catalyst promoted with potassium (100 g Fe/2 g K) is studied over a wide range of oper-ation conditions to investigate its suitability to produce C2C4 components from H2/CO2 mixtures. Theachieved hydrocarbon distribution ( = 0.20.3) allows for the production of Substitute Natural Gas com-ponents (68 C% C1, 30 C% C2C4, C5+ approx. 2 C%). The catalyst stability is good, at least for 50 days.The hydrocarbon selectivity remains almost constant during the experiment, methane becoming slightlymore predominant over time. Catalyst activity seems to be strongly inuenced by the (H2O/H2)out ratio(possibly due to oxidation), which correlates with CO2 conversion. At high values of (H2O/H2)out, theactivity of the catalyst seems to change and cannot be described using the same reaction rate kinet-ics determined for lower (H2O/H2)out values. The maximal CO2 conversion achieved is 44% (p = 2 Mpa,(H2/CO2)in = 8). Experimental results show that higher conversions could not be achieved neither withan increase in temperature nor in modied residence time. The H2/CO2 inlet ratio is the most promisingparameter to reach high CO2 conversions without a high oxidation potential in the product gas. Interest-ing catalytic effects have been identied, however experimental results will be supported by additionalwork in order to get a better understanding of the CO2 hydrogenation under FischerTropsch conditionswith iron catalysts.

2014 Elsevier B.V. All rights reserved.

ction

easing contribution of renewable electrical energy toergy supply involves, due to its uctuating production,ment of new storage technologies to stabilize the elec-Chemical energy carriers, e.g. gaseous hydrocarbons,

energy density and a developed infrastructure for stor-ution and utilization are seen as an important option- and long-term storage [1,2]. The production of chem-om electrical energy may also become relevant in a

renewable energy system. Wind and solar energy offer

ding author. Tel.: +0049 721 608 42575.ress: maria.iglesias@kit.edu (M. Iglesias G.).

the highest potential to produce energy from renewable sourceswithout constraints and probably hydrocarbon fuels will remainrelevant for process heat (gas) or transport (liquid, e.g. for airplanes,ships) (Fig. 1.1).

Synthesis of gaseous hydrocarbons using CO2 as C-source hasrecently gained special interest since it may be attractive to produceH2 using excess electrical renewable energy via water electrolysis,a proven electrochemical technology at small scale. The availabilityof CO2 is high and CO2 may be directly separated after its formationin corresponding industrial processes. Potential sources of concen-trated CO2 are biogas facilities, biomass combustion or gasication(leading to CO/CO2 mixtures) and blast furnace offgases. The useof CO2 as carbon source offers the possibility to replace naturalgas or compounds from petroleum rening (liqueed petroleumgas, LPG), thus avoiding fossil CO2 emissions. Methane is the most

rg/10.1016/j.cattod.2014.05.0202014 Elsevier B.V. All rights reserved.al energy storage in gaseous hydrocarbTropsch synthesis from H2/CO2Kines considerations

as G.a,, C. de Vriesb, M. Claeysb, G. Schauba

-Institut, Karlsruhe Institute of Technology, Engler-Bunte-Ring 1, 76131 Karlsruhe, Germtre of Excellence in Catalysis, c*change, Department of Chemical Engineering, Universits via iron, selectivity and

pe Town, Private Bag X3, Rondebosch 7701,

M. Iglesias G. et al. / Catalysis Today 242 (2015) 184192 185

Fig. 1.1. G

Table 1High heating v

MJ/m3

common gatute Naturawidely studet al. [4]). Ithigh conver

Short chincrease theing value) =gas grid (H-has been fo[5,6], most short chainhydrocarbovious studicatalytic reaCO2 is convwards, CO rproducts (Eextend in-s

CO2 + H2 nCO + 2 nHCnH2n + H2

The aimlimits of a nthe producttures. FundFT conditionprocess con

2. Experim

2.1. Iron ca

Hydrogesystems ba(Ni), Ruthedard methaC4 hydrocaralysts has nhigh H2/COis observedfor Ru-baseactive for musually no Riedel and the reactanare well kn

actorydro

, in orsis gpertO2/C

cataydro

ium ( cataters er ad

reaco hasre ces [1

lowtion ts ofs. Ths on

expe ironreci

e(NO3)39H2O) with an aqueous NH3 solution (5 wt.%) atd under vigorous stirring (24,000 rpm). After washing with

zed water the catalyst precursor was dried for 12 h at 120 Cnd crushed to fraction below 125 m. This was followed bytion under airow (900 ml(STP)/gFe/min) in a uidized bedr at 400 C for 3 h (heating rate: 10 C/min). The catalyst waseneral ow diagram of electricity conversion to chemical fuels.

alue (HHV) of substitute natural gas components at STP (105 Pa, 0 C).

CH4 C2H6 C3H8 C4H10

39.8 69.6 98.7 127.5

seous hydrocarbon and the main component of Substi-l Gas (SNG). Hydrogenation of CO2 to methane has beenied in literature (e.g. Wang and Gong [3]; Kopyscinski

is usually carried out with a nickel-based catalyst andsions and selectivity to methane can be reached.ain hydrocarbons, (C2H6, C3H8, C4H10) are used to

caloric value of methane (raw SNG) (HHV (high heat- 39.8 MJ/m3) before being fed into a high caloric valuegas) (HHV = 4047.2 MJ/m3, Table 1). Little informationund in literature for this route using H2/CO2 mixturesof the research has been focused on the production of

alkenes [7,8] for the chemical industry or long chainns [911] for transportation fuels from H2/CO2. Pre-es describe the hydrogenation of CO2 as a two-stepction, with CO as intermediate product. In the rst steperted to CO in the CO2-shift reaction (Eq. (1)). After-eacts in the FischerTropsch reaction to form organicq. (2)). The alkenes can be hydrogenated to a variableitu to alkanes (Eq. (3))

CO + H2O (1)

2 CnH2n + nH2O (2) CnH2n+2 (3)

of the present study is to investigate the potential andew practical application of FischerTropsch synthesis,ion of short chain hydrocarbons from H2/CO2 gas mix-amental catalytic effects of CO2 hydrogenation unders with iron catalysts should be identied and differentsiderations evaluated.

ental work and methodology

talyst

nation of CO2 can be conducted mainly with catalytic

bed rechain haturessyntheing proboth C

Thechain hpotassoverallpromostrongondarybut alstherefoparticlativelypromoamouncarbondependmined

Thequick p(1 M FpH7 andeioniin air acalcinareactosed on group VIII metals (iron (Fe), cobalt (Co), Nickelnium (Ru) [3,12,13]). Ni-based catalysts are the stan-nation catalysts. The possibility to produce C2, C3 andbons when using H2/CO2 as gas mixture with nickel cat-ot been reported in literature. This might be due to the

ratio prevailing in the reaction. Usually almost no CO in the nal product. Similar results have been reportedd catalyst, however, Ru is more expensive and moreethanation than Nickel [14]. Co-based catalysts have

shift activity at moderate reaction temperatures [15].co-workers [16] showed that at higher CO2 content int gas the main product is methane. Iron-based catalystsown in commercial FT synthesis [15]; the ARGE xed

impregnatedried and cconrmed b

2.2. Experim

CO2 hydbed reactoris lled wit(dp = 1001the catalystfer limitatiodrop acrossFig. 2.1. Lab scale set up with xed bed reactor.

operates at low temperatures in order to produce longcarbons (wax) and the synthol reactor at high temper-der to produce gasoline and alkenes, in both cases fromas (H2 + CO). Fe-based catalysts show the most promis-ies for the CO2 hydrogenation, due to their activity forO shift and FT synthesis (Eqs. (1) and (2)).lyst selected in this study for the production of shortcarbons from H2/CO2 mixtures is iron, promoted withK). K has probably the most signicant inuence on thelyst performance in comparison with other common(e.g. Mn, Cu). It increases the shift activity, due to thesorption of CO/CO2 on the catalyst surface, inhibits sec-tions, favors the production of long chain hydrocarbons

a tendency to lead to carbon deposition on the catalyst,ausing deactivation and disintegration of the catalyst5,17]. The ratio Fe/K selected here is 100/2(g/g). The rel-

concentration of potassium was chosen because alkaliis benecial for CO2 hydrogenation, however, larger

K would favor the production of longer chain hydro-e optimal degree of potassium promoting the catalyst

the reactant gas composition, and needs to be deter-rimentally [18].

catalyst used in the present study was prepared throughpitation from hot (95 C) aqueous iron nitrate solutiond with an aqueous 1 molar K2CO3 solution and againalcined. The catalyst composition was 100 g Fe: 2 g K asy means of AAS analysis.

ental set-up

rogenation is studied in a continuous lab-scale xed- (Fig. 2.1). The glass reactor tube (di = 8 mm; L = 700 mm)h catalyst particles (dp = 50100 m) diluted with SiO260 m) in a weight ratio catalyst to SiO2 = 1/3. The size of

particles was chosen to avoid intraparticle mass trans-ns [19] and the bigger SiO2 particles reduce pressure

the catalyst bed (the catalyst particles cover the larger

186 M. Iglesias G. et al. / Catalysis Today 242 (2015) 184192

Table 2Operation conditions for catalyst calcination and reduction.

p (MPa) Gas () H2/Ar () mod (kg s/m 3) Tstart (C) T (C min1) Tend (C) Tholding (h)Calcination 0.1 Ar 0/1 3000 25 2 200 16Reduction 25

SiO2 particachieve a uare physicaavoid particalyst allowsthe reactionproductionalyst bed isa centrally catalyst bedcatalyst macatalyst bedsystem, andpreheating frit retains ttube in ordannular spawith an O-rmetal tubecartridges ewool. The rheated in thThe volume(MFC). To cused for COconstant usafter the retom of theexpanded uthe reactorsure, T = 18downstreamof nitrogento the prodand allow tinterest frois split intograph (Agileanalyzed (e(FID) and ocolumns (MHP-5). The sampler, whhot gaseouscooling traptemperaturchromatogrumn (HP-1[18,20,21]).

2.3. Experim

Once threactor undinlet volumduring the afrom the tarThese measand average

reactl kinet

)

prodD. Thred a

cataused). Af. The. The(H2/re mions wer tx tra

ata a

molted d force g

ts). Cls. Talcult in tCO2-

out =

CO2 the o

Fn,C

CO yow

Fn,COFn,CO

yielc prot case, the organic products are all gaseous at the wax trapions.

FnC,Corg,outFn,CO2,in

(7)

yield of carbon (i.e. carbon not accounted for: heavy hydro-s, carbon deposited on catalyst) is determined from the

balance as the difference between carbon molar ow in the0.1 H2/Ar 1/3 3000

les as an adhering layer). In this way it is possible toniform gas ow at low pressure drop. Catalyst and SiO2lly mixed and afterwards distilled water is added tole segregation during reactor lling. The dilution of cat-

almost isothermal operation (13 C, depending on temperature) due to the reduction of thermal energy

per volume unit. The temperature prole along the cat- measured with a moveable thermocouple inserted inpositioned thermo well (dext = 4 mm). The length of the

used varied between 10 and 20 cm depending on thess used. A bed of inert particles is placed below the, to reach the proper vertical position for the heating

on top of the catalyst bed, to serve as gas mixer andzone. On the bottom of the glass reactor a porous glasshe particles. The glass reactor itself is placed in a metaler to allow experimentation at higher pressures. Thece between the glass and the metal reactor is sealeding to prevent gas recirculation and bypass effects. The

is heated via three alumina blocks with four heatingach (L = 200 mm, dext = 6 mm) and insulated with glasseactant gases, H2 and CO2 (purity > 99.995%), are pre-e feed line up to 250 C and enter the reactor at the top.tric gas ows are controlled by mass ow controllersover a wide range of volumetric ows, two MFCs are2 and two for H2. The reactor pressure is maintaineding a pressure-regulated argon ow, which is addedactor outlet. Pressure is measured at the top and bot-

reactor. The product gases and the added argon aresing a manual control valve and a needle valve. After, the product ows through a wax trap (reaction pres-0 C) where long chain hydrocarbons condensate. All

tubing is electrically heated at T = 200 C. A mixture (N2) and cyclopropane (Cpr) (0.5 vol% Cpr) is addeduct ow; these compounds serve as reference gaseshe calculation of molar ows of various compounds ofm gas chromatographic analyses. The product gas ow

two. The rst ow goes to the online gas chromato-nt 7890A GC), where the product gases are sequentiallyach 25 min) by means two ame ionization detectorsne thermal conductivity detector (TCD) using variousolecular sieve 5A, Hayesep Q, Hayesep T, GS-Gaspro,

other part of the product ow is routed to an ampouleere pre-evacuated glass ampoules can be lled with the

product every minute. After this the gas ow passes a, where water and hydrocarbons condensate at roome. Ampoules are afterwards analyzed in an ofine gasaph which is equipped with non-polar capillary col-) and an FID (example chromatograms can be found in

ental procedure

e reactor is loaded, H2 and CO2 ow through the colder pressure is set in order to check the inlet ratio and theetric ow for each condition that is going to be used laterctual experiment (deviations need to be lower than 2%

Table 3Range ofof globa

mcat (g

12

of the the TCmeasu

Thewater Table 2sphere300 Cset to tions acondit10 h loThe wa

2.4. D

Thecalculamethoreferenponenintervalet is cproducin the

Fn,H2O,

TheCO2 at

XCO2 =

Themolar

YCO =

Theorganipresencondit

YCorg =

Thecarboncarbonget value, measurements are done with the online GC).urements are repeated at the end of the experiments

values are used in the data analysis. The composition

inlet and ou

YC = XCO2 2 400 16

ion conditions set in experiments with 100 g Fe/2 g K for determinationic parameters.

p (MPa) T (C) (H2/CO2)in () mod (kg s/m3)12 245320 48 1005000

uct mixture needs to be within the calibrated ranges ine total volumetric ow in the outlet of the test rig isfter adjusting the position of the throttling valve.lyst is dried in an argon atmosphere to evaporate the

to mix the catalyst with the SiO2 (conditions seeter this drying, the catalyst is reduced in an H2/Ar atmo-

reactor is then cooled to the rst reaction temperature, pressure of 1 MPa is adjusted and the reactant gas owCO2)in = 5, mod = 400 kg s/m3. Initial operating condi-aintained for several days (Fig. 3.1). After steady stateare reached (CO2 conversion standard deviation withinhan 1%), new reaction conditions are adjusted (Table 3).p and the cooling trap are emptied daily

nalysis and denitions

ar ows of the organic and inorganic components arebased on the peak areas obtained with the GC (same

online and ofine GCs) related with the peak area of theas (N2 for inorganic components, Cpr for organic com-alibration factors for the TCD are determined at regularhe molar ow of water vapor, Fn,H2O,out, at reactor out-ated based on the molar ows of CO and C in organiche outlet using the stoichiometric coefcients of watershift (H2O,CO2sh) and FT reaction (H2O,FT):

FnC,Corg,out (H2O,CO2-sh+H2O,FT

)+ Fn,CO,out

(H2O,CO2-sh

)(4)

conversion is calculated based on the molar ows ofutlet and inlet of the reactor:

O2,in Fn,CO2,outFn,CO2,in

(5)

ield is the molar ow of CO at the outlet divided by theof CO2:

,out

2,in(6)

d of organic product is based on the molar ow of C inducts divided by the molar ow of CO2 (Eq. (7)). In thetlet, divided by the molar ow of CO2:

YCO YCorg (8)

M. Iglesias G. et al. / Catalysis Today 242 (2015) 184192 187

Table 4Reaction rate constants and inhibition parameters for Eqs. (11) and (12) determined based on experimental results and non-linear regression analysis.

kj (245 C) mol/s kg Pa kj (266 C) mol/s kg Pa kj (286 C) mol/s kg Pa aj () bj () EA,j kJ/mol k0 mol/s kg PaCO2-shift 1.7 108 5.4 108 1.8 107 6FT

The modbefore reduature and p

mod =m

FV,in

2.5. Lab-sca

Mathemin this projsented. It gbetter undekinetic parawith changthe molar reactor).

The lab a plug owthat the axBo > 100) ani.e. there arsteady statenents are co

A steadyformulatedand reactiocomponentcoefcient per initial m

dFn,idz

= mcaLbe

The reac(Eq. (1) andwork by Rie

rCO2-sh = kC

rFT = kFT

The effeby introduc

log Kp,CO2s

The resu(one for eac(RungeKutential matebed and lea

2.6. Determ

The mathmine the kon experima nonlinear

Detealuesg thearam

calcun-based catalyst. Gray particles: silicon dioxide (SiO2).

eters are varied to adjust the calculated values to the experi-l results. The sum of square errors of the differences betweenerimental and calculated values (integral CO and hydrocar-

eld) is minimized. Calculations are done for variable valuesperature, pressure, modied residence time and inlet ratio2)in.

lsqnonline solver requires starting values for the parame- be optimized. Due to the strong effect of starting values, ane procedure is used, until no differences are found amongrameters calculated by two consecutive calculations. Theion parameters aj, bj are considered to be independent ofrature. In a rst step aFT, bFT are calculated using all exper-l data available and constant values for kj and ash, bsh. The

step calculates ash, bsh using the new calculated values forand constant values for kj, using all experimental data avail-e third step then calculates ksh and kFT for each temperature,

the previously calculated parameters aFT, bFT and ash, bsh1 and 2). With the new calculated kj values, the algorithms to step 1. Once the algorithm calculates the kinetic param-hat best t the experimental data, activation energy (EA)e-exponential factor (k0) are calculated using the Arrheniuson (Table 4).

erimental results, modeling and discussion

n catalyst formation7.9 107 1.7 106 3.9 106

ied residence time is the ratio of initial catalyst mass,ction, and the volumetric feed ow at standard temper-ressure (STP) (105 Pa, 0 C):

cat

,STP(9)

le reactor mathematical model

atical modeling of the lab scale reactor plays a key roleect in combination with the experimental work pre-ives the possibility to design experiments, to gain arstanding of the reaction system and to determine themeters. Due to the characteristics of xed bed reactorsing molar ows with position it is needed to integrateow of each component along the catalyst bed (integral

scale reactor used can be mathematically described as reactor. Under reaction conditions it can be assumedial dispersion is negligible (Peax = 0.5, Lbed/dp > 1000;d it is perfectly mixed in radial direction (dR,eq/dp > 10),e no signicant wall effects. The reactor is operated at

and constant temperature and pressure. All compo-nsidered to be gaseous under reaction conditions.-state, one dimensional material balance equation is

for each component i (convection term in axial directionn term, Eq. (10)). Fn,i is the axial molar ow in mol/s of

i, z is the axial direction (m), ij is the stoichiometricof component i in reaction j and rj is the reaction rateass of catalyst in mol (s kg)1.

t

d

2j=1

ijrj (10)

tions considered are the CO2-shift and the FT reaction (2)). The kind of rate equations used is based on earlierdel et al., Eq. (11) and (12) [9].

O2-sh pCO2 pH2 (pCO pH2O)/Kp,CO2-sh

pCO + ash pH2O + bsh pCO2(11)

pCO pH2pCO + aFT pH2O + bFT pCO2

(12)

ct of the chemical equilibrium limitation is considereding the chemical equilibrium constant [22]:

h = log(

pCO2 pH2pH2O pCO

)=(

2073T(K)

2.029)1

(13)

lting system of ordinary differential balance equationsh component) is solved by means of numerical methodsta solver, implemented in Matlab (ode45)). The differ-rial balance equations are integrated along the catalystd to calculated values for outlet gas ows.

ination of kinetic parameters

ematical model of the lab-scale reactor is used to deter-

Fig. 2.2.mental vbed usinkinetic pbetweencles: iro

parammentathe expbon yiof tem(H2/CO

Theters toiterativthe painhibittempeimentasecondaFT, bFTable. Thusing (steps returneters tand prequati

3. Exp

3.1. Iroinetic parameters (kj, aj, bj) for both reactions, basedental data (Fig. 2.2). This procedure is iterative using

least square solver (lsqnonline, Matlab). The kinetic

The formin a molar r1 MPa. Dur128 0.8 138.9 1.6 10152 0.5 94.6 2.6 106

rmination of kinetic parameters, schematic. White symbols: experi-. Curves: molar ows for each compound calculated along the catalyst

mathematical model with the given kinetic parameters. The set ofeters that best describe the catalyst activity minimize the differencelated and experimental values at the end of the reactor. Black parti-ation of the iron catalyst takes place with H2 and CO2atio 5:1, reaction conditions are T = 297 C and pressureing this formation the metallic iron, obtained via the

188 M. Iglesias G. et al. / Catalysis Today 242 (2015) 184192

Fig. 3.1. Formin C% hydrocarmod = 400 kg s

reductive piron carbideyield of COmined by cto Eqs. (1) produced vFT reactionlow (approprevail in thcient to pslight decrebe observedorganic yierst minuteuct distribuhydrocarbomain compof oxygenat0.3 ppm), w

Schulz [of carbon fothe availabioptimal degof K in the cintermediaFT conditiobe expectedtions is relathe formatilyst formatthat almostH2/CO ratiorelatively haccount forC24 and 8 rst 100 milic iron, whreactions. Tsteady statthe time it t

The steady state value for hydrocarbon selectivity is approachedwithin 100 min (Fig. 3.1). Hence, selectivity approaches the steadystate values faster than activity. Hydrocarbons can be detected inthe product since the very beginning.

formion clightld in(19 C

prodC%). vity ie unt at onclut no

talys

stabions nde

usly owedw d

ends.2 anhanen C2it coualyst

talys

effeted iganicrage

or ea

Effectperaamication of iron catalyst as function of time, CO2 conversion and yieldsbon product distribution in C%. Conditions: T = 297 C, (H2/CO2)in = 5,/m3, p = 1 MPa.

re-treatment, converts, at least partially, into various and oxide phases [23]. Fig. 3.1 presents CO2 conversion,, yield of organic products and yield of carbon (deter-arbon balance, Eq. (8)) as a function of time. Accordingand (2), CO appears as an intermediate product, it isia the CO2-shift reaction and reacts afterwards in the. The yield of CO calculated during activation is veryx. 4 C%), which indicates low partial pressures of COe reactor. Nonetheless these CO concentrations are suf-roduce hydrocarbons via FischerTropsch synthesis. Aase of the FT activity indicating some deactivation can

over time (slight increase in CO yield and decrease inld). Hydrocarbon selectivity is investigated during thes of synthesis using the ampoule technique. The prod-tion, calculated in C%, is rich in methane and short chainns, with a very small content of C5+. Alkanes are theounds in the C24 fraction (93 C%). Very low amountses (alcohols), mainly C (approx. 2 ppm) or C (approx.

Theoperat3:1. A sCO yielower lighterC5+ (6 FT actiAlso thcontenAs a cpresen

3.2. Ca

Thecondittivity uprevionot alland sloyield t(Figs. 3in metfractiotivity, the cat

3.3. Ca

Thepresenand orthe aveerror f98%.

3.3.1. Tem

modyn1 2ere also detected in the product.18] recently proposed that potassium controls the ratermation through CO-decomposition in iron catalyst andlity of CO for carbon-on-site formation determines theree of alkali promotion. In the present work the amountatalyst is low, and also the molar fraction of the reactionte CO is very low (YCO < 1 vol%), i.e. far away from typicalns (approx. YCO = 33.3 vol%, using (H2/CO)in = 2). It can

that the formation of carbide phases at these condi-tively slow [23]. An important parameter to evaluateon of iron carbides is the carbon yield. During cata-ion, the carbon yield is very low, (

M. Iglesias G. et al. / Catalysis Today 242 (2015) 184192 189

Fig. 3.2. Effect of temperature on conversion, yield and hydrocarbon selectivity in C%. Left: White symbols: TOS 6500 min (4.5 days). Gray symbols: TOS 46,00055,000 min(32 to 38 days). YC approx. 4%. Curves calculated with kinetic model. Hydrocarbon selectivity in C%. Right: conversion and yields as a function of time on stream (C%). Shadedarea: average values cannot be calculated, deactivation observed. Conditions: mod = 1000 kg s/m3, (H2/CO2)in = 5, p = 1 MPa.

Generally literature reports a decrease in the chain growth prob-ability with increasing temperature, under typical FT conditionswhere H2 and CO are used as a synthesis gas [15]. At higher temper-atures, desorption of hydrocarbon products is favored. The effectof temperatas the carboknown to hof higher lecontribute t

3.3.2. EffectThe CO2

modied reis characterthe interme5000 kg s/mnot show anto the highreaction an

The hydrocarbon product distribution is shifted to longerchain hydrocarbons with increasing modied residence time(2003000 kg s/m3). The chain growth probability increases from0.20 to 0.32. At a residence time of 5000 kg s/m3 and probably

deac of ttant d toin hi28]. furthitionsamevity c

Effect H2/Cnt

cas

Fig. 3.3. Effectrepeat experimmodel. Valuesure could be masked in this case, where CO2 was usedn source by the decrease in the H2/CO ratio, which is

ave a positive effect on chain growth [15]. The presencevels of water vapor at high temperatures may furthero enhanced chain growth [28].

of modied residence timeconversion and the hydrocarbon yield increase withsidence time (Fig. 3.3) and the CO yield decreases as itistic for reactions that occur in series, with CO beingdiate product. At high modied residence times (i.e.3), the CO2 conversion and yield of organic carbon do

increase, even a decrease was observed, probably due partial pressure of water vapor that may inhibit thed even cause catalyst oxidation [16,2426].

due tochangeof reacresponresult bility [which in addin the selecti

3.3.3. The

(constathan in of modied residence time on CO2 conversion, and yields of hydrocarbon products and ent after 30,000 min (21 days) of testing. Shaded area: deactivation observed, kinetic p

in C% Right: Hydrocarbon selectivity in C%. Right top: C% of alkene in the fraction C24. Cotivation, the methane selectivity increases again. Thishe product distributions can be explained with changeand product partial pressure. Low residence times cor-

low water vapor partial pressure, which is known togh methane selectivity and low chain growth proba-At these conditions also the H2/CO ratios are larger,er contribute in this regard [27]. Catalyst deactivation,

to phase changes that may affect selectivity, results changes of partial pressures and therefore the samehanges.

of H2/CO2 feed ratio and reaction pressureO2 inlet ratio is varied at constant feed volumetric ow

mod). Therefore at ratio 8, the molar ow of CO2 is lowere of ratio 4. Nevertheless it can be seen in Fig. 3.4 thatCO (in C-%) and hydrocarbon selectivity. Left: gray symbols: baselinearameters are not valid in this region. Curves calculated with kineticnditions: T = 266 C, p = 1 MPa, (H2/CO2)in = 4.

190 M. Iglesias G. et al. / Catalysis Today 242 (2015) 184192

Fig. 3.4. Effecyields of hydrsymbols p = 1 mod = 1000 kg

higher inletmost likelyup these reChoi [29]. Taddition of both reactioever the mopartial presIn addition,H2 and CO2hydrogen p

The effeFig. 3.4. A 2 MPa resuyield of orgincreases. A1.5 MPa ind

3.3.4. ModeThe And

to represeresults obtber of datditions is T = 245297ideal ASF coften obsereffects of foof incorpor

ym,i = NC,i

3.3.5. EffectA main d

carbons is H2/H2O, realysts and reaction anreaction to is relativelyis produced

. Hydn-Sch297 C

ing causn casis [tite ce ooxidaidatied byl2O3ll exnsidepH2)of orgalystdicatnverO2 cos ratieach

it ipH2)on anng itt of molar inlet ratio and reaction pressure on conversion andocarbon products and CO (in C-%). White symbols: p = 2 MPa; BlackMPa. Curves calculated with kinetic model. Conditions: T = 266 C

s/m3.

ratios favor higher conversion and hydrocarbon yields, due to the higher hydrogen partial pressure speedingactions. This behavior is conrmed in the literature byhe effect of (H2/CO2)in on CO yield is negligible. TheH2 seems to be benecial for the catalyst, it increasesn rates, has an effect on the reversible reaction, how-st interesting effect is that, due to the excess of H2, H2Osure is reduced, with a positive effect on reaction rates.

hydrocarbon selectivity is affected by the inlet ratio of. As to be expected a high ratio and corresponding highartial pressures favor the formation of methane.ct of reaction pressure at inlet ratio 4 is also shown intwofold increase of the total pressure from 1 MPa tolts in a small increase of the CO2 conversion and theanic products as with higher pressure the contact timeccording to literature, an increase in pressure beyondeed seems to have no pronounced effects [24].

ling of hydrocarbon product compositionersonSchulzFlory (ASF) model (Eq. (14)) is able

Fig. 3.5AndersoT = 245

consumwhichH2O casynthemagnepresencause The oxreportFe/K/A

If aare co(pH2O/yield othe catthis inCO2 cowith C

Thiity to rnding(pH2O/positiochangint, in a rather accurate way, the experimentalained in this study (see Fig. 3.5). A large num-a points representing a variety of reactions con-shown (conditions: p = 12 MPa, (H2/CO2)in = 458,C, mod = 2005000 kg s/m3). Small deviations froman be found in the fractions C24 and C5+, which areved experimentally and which can be explained withrmation of methane on non-FT specic sites and effectsation of primarily formed alkenes [30].

(1 )2 NC,i1 (14)

of (pH2O/pH2 )out on activity and selectivityifference between CO2 and CO hydrogenation to hydro-the partial pressure of water vapor, or the ratio ofspectively. Under typical FT conditions with iron cat-H2/CO as feed gas, water vapor is produced in the FTd, to a large extent, consumed in-situ in the CO-shiftform H2 and CO2. Consequently the ratio (pH2O/pH2)out

low. In case of using H2/CO2 as feed gas, water vapor in the FT synthesis and also in the CO2-shift reaction,

tivity. An anthe gas phaHjlund-Ni

Higher vtial. Eq. (15based on stthe organicratio (H2/COdation potethat for a feis achievedin Fig. 3.6 cmental dataof CO2 conv

(pH2OpH2

)ou

The hydhigh (pH2O/increase, prrocarbon product distribution in C%. Curves calculated withulz-Flory (ASF) model. Conditions: p = 12 MPa, (H2/CO2)in = 458,, mod = 2005000 kg s/m3.

H2. The resulting value (pH2O/pH2)out will be higher,es a more oxidizing atmosphere. It is known thatuse oxidation of iron catalysts during FischerTropsch17,25,26,31]. Hgg carbide (-Fe5C2) can oxidize to(Fe3O4), which is not active for the FT synthesis, inf H2O whereas CO2, even at very high levels, does nottion [25]. This oxidation is typically reversible [25,31].on of Fe to FeO at (pH2O/pH2)out > 0.16 has also been

Bartholomew [17] for the ammonia synthesis with ancatalyst.perimental results using the 100 g Fe/2 g K catalystred, independent of the reaction conditions, the ratio

ut correlates directly with the CO2 conversion and theanic products (see Fig. 3.6). Due to the characteristics of

and its high FT activity, the CO yield is always very low,es that the yield of organic products is very close to thesion and the partial pressure of water vapor increasesnversion.o may also be responsible of the apparent impossibil-

high CO2 conversions with this catalyst. Based on thiss suggested that there is possibly a limiting value of

ut, which is inuenced by temperature, catalyst com-d reaction conditions, at which the catalyst could starts composition and consequently its activity and selec-alogous suggestion regarding the interaction betweense and catalyst composition was presented earlier byelsen et al. [32].alues of this ratio indicate a higher oxidation poten-

) shows an approach to calculate the (pH O/pH ) ratio2 2 outoichiometry. It is assumed that the H/C ratio value in

product is 3.3. Based on this equation, the molar feed2)in is the most promising variable to decrease the oxi-ntial. Indeed the results of the present study indicateed ratio up to 8, the maximum CO2 conversion (44%)

(T = 290 C, mod = 1000 kg s/m3 p = 2 MPa). The curvesalculated with Eq. (15) localize the range of the experi-

points for two inlet ratios H2/CO2 and variable valuesersion and CO yield.

t

= YCO + 2 YCorg( H2CO2

)in

YCorg 3.65 YCorg(15)

rocarbon selectivity is also inuenced by this ratio, atpH2)out the C% of methane decreases and C24 and C5+obably due to the lower H2 availability and the higher

M. Iglesias G. et al. / Catalysis Today 242 (2015) 184192 191

Fig. 3.6. Inuence of (pH2O/pH2 )out on activity and selectivity. Left: CO2 conversion and CO yield in C

YCO = XCO2 for XCO2 < 0.03. Solid curve for XCO2 :(H2/CO2

)in

= 4; dashed curve (H2/CO2)in = 8. Right: HFull symbols black: C balance close with an error >5%. Full symbols gray: deactivation observed. White syT = 245297 C, mod = 1755000 kg s/m3.

concentrations of water vapor. This ratio seems to have a strongereffect than temperature, modied residence time or pressure.

3.4. Kinetic analysis

To deterout catalystorganic yiepared withtotal 64 expters. Operattwo differeent inlet r(1753000 be used to temperatur3000 kg s/mtime is 1000compared t

Fig. 4.1. Effecculated using curve: (H2/COsolid lines areof CO2 is const

of the relatiate CO2 conactivation ethat for the

re sebj arehibit

shift relatalyst

clus

ocess

usearbod FTtwo d. T = ediat accmine the kinetic parameters, only experiments with- deactivation and with Yc < 5% are considered. CO andlds are used as experimental results and to be com-

the values calculated by the mathematical model. Inerimental points were used to determine 10 parame-ion conditions for the experimental data vary betweennt pressure levels (1 MPa and 2 MPa), three differ-atios (4,5,8) and various modied residence timeskg s/m3). The resulting kinetic parameters should notcalculate steady state operation conditions at higheres than 286 C and modied residence time larger than3. In case of 286 C, the maximum modied residence

kg s/m3.Kinetic constants are higher for the FT reactiono the CO2-shift reaction. The yield of CO is an indicator

peratuaj and lyst exi.e. thecan bethe cat

4. Con

4.1. Pr

Thehydroction anuse of approxintermcatalyst of mcat and (H2/CO2)in on achievable CO2 conversion. Curves cal-kinetic model. Black lines calculated for a constant (H2/CO2)in; solid2)in = 4, dotted curve (H2/CO2)in = 6, dashed curve (H2/CO2)in = 8. Gray

calculated for a constant catalyst mass. Conditions: Inlet molar owant, T = 286 C p = 1 MPa.

ence of watH2O in-situconsideredbranes exhregarding Hexperimentthe low CO(H2/CO2)inseries, in boand condenversion. Cavariables tomolar ow

Considecharacteriscomponentbe co-produfor the prodtion; this le(alpha valu-%. Lines calculated with Eq. (15) with YCO = 0.03 if XCO2 > 0.03, and

ydrocarbon product distribution in C% as a function of (pH2O/pH2 )out.

mbols: stable operation. Conditions: p = 12 MPa, (H2/CO2)in = 458,

onship between shift and FT activity. Assuming moder-version, lower CO yields indicate a high FT activity. Thenergy of the CO2-shift reaction is 1.5 times larger than

FT reaction. The activation energies represent the tem-nsitivity of the reactions, because the inhibition factors

considered to be temperature-independent. The cata-s a high FT activity in relationship with the shift activity,

reaction is the rate-limiting step. The low shift activityed to the relatively low concentration of potassium on.

ion

considerations

of iron-based catalyst for the production of gaseousns offers the possibility to carry out the CO2-shift reac-

synthesis with the same catalyst. This eliminates theifferent reactors with different temperature levels (i.e.

900 C for CO2-shift and e.g. T = 250 C FT reaction) andte cooling of the product gas. However, the use of an ironounts for some challenges mainly concerning the pres-

er vapor as byproduct. The use of membranes to remove

and enhance the CO2 hydrogenation have already been by Rohde [33,34], although currently available mem-ibit low stability, permselectivities and permeances2O under FT conditions. According to the presentedal results a single pass reactor is not an option due to2 conversion values under stoichiometric conditions,= 4. Either a gas recycle or a combination of reactors inth cases with intermediate water removal (e.g. coolingsation), are alternatives to increase the global CO2 con-talyst mass and (H2/CO2)in are two important design

reach a certain CO2 conversion per pass if the inletof CO2 is xed (Fig. 4.1).ring the selectivity, it is well known that according to thetics of the FT synthesis it is not possible to produce C24s with a high selectivity. Methane and C5+ will alwayscts in the gas product. Two strategies appear attractiveuction of C24 compounds: (a) minimize the C5+ frac-ads to a larger C1 fraction with a reduced C24 fractione between 0.1 and 0.3); the presented catalyst (100 g

192 M. Iglesias G. et al. / Catalysis Today 242 (2015) 184192

Fe/2 g K) is an example for this strategy and interesting for theproduction of substitute natural gas components, or (b) maximizethe production of C24 components (alpha 0.50.7); this leads to alarger C5+ fraction, which is more valuable than methane, but needsto be separated from the product and can be upgraded to be usedas transportation fuel.

4.2. Future work and outlook

Interesting catalytic effects have been identied when using100 g Fe/2 g K for CO2 hydrogenation under FT conditions. Theresults obtained for this catalyst will be supported by additionalcatalyst characterization work required to get a better understand-ing of the catalytic effects.

Experimental results showed that it is not possible, under theoperation conditions used, to achieve high CO2 conversion values.The catalyst activity seems to change due to the increasing oxida-tion potent(100 g Fe/10CO2 converin shift acttially largerresistant wthe hydrocahydrocarbochanging thcatalyst (e.g

In-situ cand magnein changes addition, thtigated as toperation c[36].

Acknowled

This woristerium f(M. IglesiasFriedemann

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Chemical energy storage in gaseous hydrocarbons via iron FischerTropsch synthesis from H2/CO2Kinetics, selectivity and p...1 Introduction2 Experimental work and methodology2.1 Iron catalyst2.2 Experimental set-up2.3 Experimental procedure2.4 Data analysis and definitions2.5 Lab-scale reactor mathematical model2.6 Determination of kinetic parameters

3 Experimental results, modeling and discussion3.1 Iron catalyst formation3.2 Catalyst stability3.3 Catalyst activity and selectivity3.3.1 Effect of temperature3.3.2 Effect of modified residence time3.3.3 Effect of H2/CO2 feed ratio and reaction pressure3.3.4 Modeling of hydrocarbon product composition3.3.5 Effect of (pH2O/pH2)out on activity and selectivity

3.4 Kinetic analysis

4 Conclusion4.1 Process considerations4.2 Future work and outlook

AcknowledgementsReferences