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R e d u c t i o n o n S y n t h e s i s G a s Co s t s b y D e c r e a s e o f S t e a m /Ca r b o n a n

Ox y g e n /Ca r b o n R a t i o s i n t h e F e e d s t o c k

L . B a s i ni a n d L . P i o v e s a n *

Snamprogetti S.p.A. Research Laboratories, via Maritano 26, 20097 S. Donato Milanese, Milano, Italy

The costs for syngas production at low steam/carbon and oxygen/carbon ratios have been analyzfor simplified process schemes of the main syngas production technologies (steam-CO2 reformiaut other mal r eforming, and combined reforming) and different synthesis gas compositions. Tbroad ana lysis a rises from experimental indication on the possibility of preventing carbformat ion at low steam/carbon and oxygen/carbon rat ios in the feedstock by choosing appr opriate cat alyst or by intr oducing small amount s of sulfur compoun ds in th e reactan t feThe an alysis is limited to th e synthesis gas pr oduction st ep an d does not include its downstreprocesses. The resu lts indicat e th at technologies at low steam /car bon a nd oxygen/car bon r atwould have a significant positive impact on synthesis gas costs.

I n t r o d u c t i o n

The possible routes for finding a solution to t hetechn ological an d economical cha llenge of the exploita-

tion of natural gas (NG) include the study of syngasproduction costs reduction (Gradassi and Green, 1995).Indeed syngas is an intermediate and its production

is a demanding step which absorbs 60-70% of the costsof NG convers ion in to fina l products. A reduction of 25%of syngas production costs not only would result in astrong improvement of the existing technologies but alsowould allow the NG conversion routes to become muchmore competitive with oil refining (Lange and Tijm,1996).

The well-established a nd reliable syngas productiontechnologies have been continuously improved in thepast 40 years; comprehensive reviews of existing pro-cesses are reported in references (Rostrup-Nielsen(1984a) and Dybkjaer (1995)).

Most of energetic consumption and investment costin the syngas production technologies are related to theneed of avoiding the carbon formation reactions (1) and(2):

These reactions a re inh ibited by a n appropriate choiceof catalyst and by feeding the reactors with reactantmixtures in which high values of O2/C a nd/or S/C a r emaint ained (typically O2/C ) 0.65 v/v for au totherma land combined reforming and S/C > 2.5 v/v for steam

reforming). Rostru p-Nielsen (1993), Rostru p-Nielsen etal. (1986), and Gadalla and Sommer (1989) have dis-cussed the point in relation to steam-CO 2 reformingcatalyzed by a Ni-based cata lyst.

This paper examines the effect of different composi-tions of feedstock in well-established processes onsynthesis gas costs. The ana lysis has been suggested

by a number of experimental indications on the psibility of preventing carbon formation at low steacarbon (S/C) ratios by choosing an appropriate nometal based catalyst or by introducing small amou

of sulfur compounds into the reactant feed.Many l i terature works discuss these points; in p

t i cul ar t hi s s t udy ha s been s t im ul at ed by pr eviresearch performed by Richardson and Paripatyad(1990) and more recently by Basini and Sanfil ip(1995) an d by Rostrup-Nielsen a nd Bak-Hansen (19on noble meta l based cata lysts a nd by the discussionsulfur passivated reforming technology reported Rostrup-Nielsen (1984b), Udengaard et al. (1992), aDibbern et al. (1986).

For the purposes of this work we will assume ththe chemistry of steam-CO 2 reforming of NG whomain component is th e methane molecule could described with the simple equat ions (3) an d (4), whs ugges t t hat l ow S/C feeds s houl d be par t icul aconvenient when low H 2/CO synga s is nee de d. Nevtheless, the analysis has been extended to the prodtion of synthesis gas mixtures in which the H 2/CO r ahas been varied between 0.88 and 5.1 v/v, finding tthe lowering of S/C also reduces the costs of syngas whigh H 2 content.

Other syngas pr oduction processes which have beexamined in this study are the autothermal and cobined reforming. Equa tion 5 combined with eqs 3 a

4 can be t aken t o r epr es ent t he chem i s t r y of t heprocesses:

In t hese cases the cost var iation a nalysis has consered the possibility of reducing both S/C and oxygcarbon (O2/C) r a t ios a t ver y low va lu es , not fea siblethe existing plants.

The economic analysis has been focused on doincomparat ive study rather than on estimating absolnumbers and, due to its broadness, it has been limi

* Au t h or t o w h om corres p on d en ce i s ad d ress ed. T el .:+3 9 (2 ) 5 2 05 6 87 0. F a x : +3 9 (2) 5 2 05 6 7 57 . E -mai l:laura .piovesan@snam pro.atlas.it.

E-mail: luca.basini@snam pro.atlas.it.

CH 4 fC + 2H 2 (1)

2C O fC + CO 2 (2) CH 4 + H 2O fCO + 3H 2

CH 4 + CO 2 f2C O + 2H 2

CH 4 +1/2O2 fCO + H 2

25 8 Ind. E ng. Chem. Res. 1998, 37 , 25 8-26 6

S0888-5885(97)00402-8 CCC: $15.00 1998 American Ch emical SocietyPublished on Web 01/05/1998

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to the synthesis gas production step regardless of theimpact on the economics of the downstream users.

I n par t i cul ar i t i s m ent ioned t hat t he fol low ingassum ptions ha ve been a dopted in order to disenta nglethe effects of S/C a nd O2/C va r ia t ion s on synga s cost sfrom th e effects of other variables: (a) plant capacityhas been maintained constant at 300 000 Nm 3/h for ea chexamined case; (b) for each considered technology(steam reforming, au totherm al reforming, an d combinedreforming) reactor exit temperatures have been com-prised in a T range less than 50 C, and maximumtemperature values have been maintained below thevalues used in proven technologies; (c) NG has been

assumed to contain 100% of methane; (d) the syngaschemistry ha s been assum ed an equilibrium chemistry.The synthesis gas is an intermediate of many pro-

cesses and we m ention t he h ydrotrea ting, the synthesisof ammonia, methanol, acetic acid, and dimethyl car-bona te, the Fischer-Tropsch process, the h ydroformyl-ation pr ocess, and the reduction of iron ores.

Ea ch of th ese downstrea m pr ocesses presen ts specificdemands with r egard to (a) H2/CO r at ios, (b) pr es en ceof iner t species, an d (c) pre sence of CO2. For this reasonan economic analysis of the integrated synthesis gasproduction and its downstream utilization would havebeen too broad. However, since the syngas investmen tcosts represent the 40-70% of overall costs of th e a bove-men tioned processes, this economic ana lysis can still be

considered of some relevance to t he overall evalua tionof the syngas production-utilization technologies.

The economic analysis has been extended also to casesin which the lowering of S/C and O 2/C r a t ios h a s lefthigh CH 4 (10% v/v) residue in the product syngas. Thishigh CH 4 residue is t oo high for m an y syngas u tilizat ionprocesses but it could be tolerated by innovative large-scale processes for the exploitation of NG sources whichcould utilize Fischer-Tropsch technologies operat ingwith relatively low H 2/CO r at ios and wit h low lim it at ionon the CO2 and C H 4 content.

In conclusion it is mentioned that in recent years aninnovative cata lytic par tial oxidation (CPO) process ha sbeen proposed by Hickmann and Schmidt (1993) and

Choudary et al. (1992), who discovered the possibilityof producing syngas from prem ixed feedstock of oxygenand hydrocarbons at very high space velocity values(GHSV 500 000 h-1 or higher). Although extremelyinteresting, the CPO process will not be discussed inthe present analysis, but we mention that recently Daveand Foulds (1995) have published an economic evalu-ation devoted to methanol production with CPO.

P r o c e s s D e s c r i p t i o n a n d B a s i s f o r E v a l u a t i on s

A process engineering simulator (Aspen Plus) ha sbeen used to model three simplified process schemes(see F igures 1-3) for the production of syngas viareforming of natural gas.

The simplified cost estimation analysis lays on following assumptions: (a) th e natu ral gas u sed bas feed and fuel is composed by pure methane availaat a mbient temperatu re and at the operating pressu(b) oxygen (99.5%) is produced at operat ing pressu rean air separation unit included in the inside battlimits (ISBL) costs and its consumption of power acooling water are accounted for; (c) process waterpurchased at operating pressure and saturation teperature a nd preheated in a boiler to the temperaturequired; (d) the CO2 recycle compressors are steadriven turbines; (e) catalyst costs have not been cluded: (f) interest rate of 15% over 10 years in

production cost, for capital recovery is assumed.Detailed heat a nd ma terial balance simulat ions ha

been performed for all the cases studied. On th e baof the pr ocess schemes of Figures 1-3, process equm ent has been dim ens ioned, and i t s cos t has bevaluated on t he basis of in-house data an d Aspen Pinternal database values. The total investment cohave been calculated referring to th e total ma in equm ent i nst al led cos t s . R aw m a t er ial s ( nat ur al gprocess wat er, oxygen, an d CO 2 makeup) an d uti l itconsumption have been determined from process cculations. Raw mat erials an d th e ut i li t ies unit cr elat e t o t w o differ ent s cenar ios : U .S. G ulf C o( nat ur al gas ) 1.8 USD/MMBtu) a nd Saudi Ara(natural gas ) 0.5 $/MMBtu). In t his second scenathe investment has been multiplied by a location facof 1.2.

S t e a m-CO 2-R e fo r m in g P r o c e ss S c h e m e . Tscheme is reported in Figure 1. Natur al gas is miwith the required quantity of satur ated steam and Cand preheated to 600 C in the convective section of r efor m er . T he gas m i xt ur e i s t hen fed i nt o heareformer tu bes filled with cata lyst in the ra diant sectof the reformer at an operating pressure of 28 kg/cmThe hea t du ty for t he endother mic reaction is supplvia th e complete combust ion reaction of fuel nat ura l gThe product syngas leaves the reformer at a tempeture n ot h igher t han 900 C. Heat is recovered frthe reactor exit stream by raising high a low-pressu

F i g u r e 1 . Process flow diagram for steam-CO 2 reforming.

F i g u r e 2 . Process flow diagram for autothermal reforming.

F i g u r e 3 . Process flow diagram for combined reforming.

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stea m. The synga s is then cooled to 50 C with coolingw at er and, w hen necess ar y, C O2 i s r em oved by anabsorption/stripping step using a MDEA solution. Therecovered CO2 is compressed to the operating pressur eand recycled to th e reformer. The reformer has beensimulated as a nonadiabatic equilibrium reactor (RG-IBBS) in which the composition of the syngas corre-sponds to equilibrium values at the reaction conditions.A sensitivity an alysis h as been per formed consideringS/C ratios in the feed in the range 0.6-3.6 and H 2/COratios in the product gas from 0.88 to 5.1.

T he C O2 needed in the feed is partly obtained byrecycling the CO2 washed from the product gas to th ereformer. The two following alternatives have thenbeen considered for additional sources of CO2: (a) make-

up CO2

is available free in the plant; (b) mak e-up Ci s r ecover ed fr om t he r efor m er fl ue gases and recovery section is included in t he ISBL of the plan

Au t o t h e r m a l -R e f o rm i n g P r o c e s s S c h e m e . Tpr ocess s chem e i s s how n i n F i gur e 2. N at ur al gsaturated steam, and CO2 are preh eated to 300-500and mixed in the reactor burner at a pressure of 30 cm 2. T he aut ot her m al r efor m er i s s i m ulat ed as adiabatic equilibrium reactor, and the compositionthe syngas produced is the one corresponding to eqlibrium at the exit temperature (900-980 C) detm i ned by t he adi abat i c heat bal ance, i ns of ar as tcurrent technological limits for mat erials allow. Tassum ptions on the hea t recovery and th e CO2 remosections are the same as those described for the steam

T a bl e 1 . S t e a m-CO 2 R e f o r m i n g : C a s e s S t u d i e d ( C a p a c i t y o f 3 0 0 0 0 0 N m 3/h o f a n H 2 + C O M ixt u re)

T a bl e 2 . I n v e s tm e n t a n d P r o d u c t i o n C o s ts f o r S t e a m-CO 2 R e f o r m i n g

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CO 2-reforming process. Oxygen consu mpt ion dependson t h e h ea t d u ty t h a t h a s t o b e p rovid ed for t h eendothermic-reforming reaction and on the adiabaticexit temperatures necessary to keep the percentage ofmetha ne in the output stream a t compara ble low values(below 2.5% dry basis) in each examined case.

C ombined-R eforming Process Scheme. The com-bined reforming (Figure 3) features a combination oftubular reforming and autothermal reforming: the gasthat leaves the primary reformer at a temperature of 700 C and with significant unconverted methane ismixed with oxygen a nd fed t o the secondary r eformer.At t he exit of the secondar y reformer the heat recoverya n d CO2 r e mova l s ect ion s a r e t h e s a m e a s t h os edescribed previously.

R e s u l t s a n d D i s c u s s i o n

S t e a m-CO 2 R e f o r m i n g ( S R ). T abl e 1 l i s t s t hecases studied, with reference to a capacity of 300 000Nm 3/h of a m ixt u r e cont ain in g only H 2 + CO.

The case SR/A of Table 1 h as been referred to as t heHTAS SPARG process discussed by Dibbern et al .(1986). This process can operate at S/C ) 0.6 v/v toproduce a syngas with H 2/CO ) 0.88 v/v.

The cases SR/I an d SR/J of Table 1 refer to S/C rat iosin t he feed r espectively of 3.6 a nd 2.8 v/v to produce asyngas with H 2/CO ) 5.0 v/v. These two cases lead toa syngas composition with a stoichiometric number SN(SN see eq 6) of 4.7, which is typical of existing plantsfor methanol production (see for instance the work byDybkjaer (1995) for discussion of relat ionsh ips betweenS/C and H 2/CO a nd SN ):

In the cases SR/A-H of Table 1 the desired H 2/COra tios are pa rtly obtained by recycling to the productionsection the CO2 washed from t he reformer exit str eam.

In the cases SR/I and SR/J, instead, there is no needfor CO2 recycle a nd CO2 removal from the productstream is not included.

Table 2 reports the investment and the productioncosts estimated for the examined steam-CO 2-reformingcases.

The costs S R/A-J ha ve been estima ted for a U .S. GulfC oas t s cenar io at a nat ur al gas pr i ce of 1.8 U SD/ MMBtu. The costs of the cases SR/C, SR/F, an d SR/Ghave a lso been estimated for a Saudi Arabia scenarioat a natu ral gas price of 0.5 USD/MMBtu. Due to thewide variation in th e H 2/CO r a t ios we h ave ch ose n t o

express the production costs both on a volumetric aon a weight basis. The volumet ric basis production coar e referred t o a capacity of 300 000 Nm3/yea r wh ile weight basis production costs are referred to a capacof 1.141 MMt/year . As alrea dy mentioned, the capacconsiders only pure H 2 + C O m ixt ur es . T he r es ureported in Table 2 referring to the alternative of mau p C O2 recovered from flue gas have been plot

T a b l e 3 . A u t o t h e r m a l R e f o r m i n g : C a s e s S t u d i e d ( C a p a c i t y o f 3 0 0 0 0 0 N m3/h o f a n H 2 + C O M ixt u re)

F i g u r e 4 . Normalized production cost of synthesis gas fsteam-CO 2 reforming vs S/C ra tio in th e feed. The points r efeth e d a ta o f T a b le 2 c a lc u la te d a s s u m in g th a t m a k e u p COrecovered from flue gas. The lines result from the regression

th e d a ta o f T a b le 2 c a lc u la te d a s s u m in g th a t m a k e u p COavailable free. The pa ram eters of eq 7 ar e (a) volumetric basiss) 0.86, m 1 ) 1.14, b ) 5.51; (b) weight ba sissm 2 ) 1.1, m 1 ) 1b ) 64.68.

SN ) (H 2 - CO 2)/(CO + CO 2) (6)


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(points) in Figure 4a,b in terms of normalized productioncos t a s a fu n ct ion of H 2/CO a n d S/C r a t ios . Th enormalized production costs calculated assuming thatCO 2 is available free (Table 2) have been r egressed bymeans of eq 7.

(F2 > 0.9) and the resulting exponent ial curves plottedin Figure 4a,b (lines).

In all the examined cases for a given H 2/CO r at io, t h eproduction costs a re higher tha n t hose for the S/C rat ios.For instance, the t hree cases at H 2/CO ) 2 (cases SR/C, SR/F, an d SR/G in Ta bles 1 and F igure 4a) show tha tlowering the S/C value from 3 to 1 reduces the volu-metric production costs 23%, in the assumption thatmake-up CO2 is available free in the plant, and 21%,when accounting for make-up CO2 recovery costs. Thisvariation is relat ed for t he 60% to a reduction of variablecosts (lower pr ocess st eam is needed) an d for th e 40%to a reduction of quantit ies related to t he investment.The investmen t costs are also reduced by 26% (23% ifCO 2 recovery costs are included), and this reduction is

mainly related to the costs of the removal and comprsion of CO2. Indeed, at lower S/C ratios the amountCO 2 required to produce syngas with H 2/CO ) 2reduced according to equilibria (3) and (4).

The production costs reduction lowering the S/C vafrom 3 to 1 r ises from 23 t o 26% when considering location Sau di Ara bia. This difference origina tes frthe lower incidence of variable costs on the productcost (natural gas price ) 0.5 USD/MMBtu) together wan increase in capital cost due to a location factor1.2.

However the data of Table 1 also indicate that syngproduced with S/C ) 1, even though more economicconta ins a relevant amoun t of un converted CH 4 (=1on a dry gas basis) while the cases with S/C ) 2 anproduce a syngas with a much lower almost the sapercentage of CH 4 (=3% on a dry gas basis). To alcomparisons between cases in which methane convsions are different, the syngas capacities in termspure H 2 + CO mixtures have been maintained the sa(namely 300 000 Nm 3/yea r or 14 1.3 MMt /year ). Italso noted that the CH 4 percentage in t he exit s trewould ha ve been reduced by simply raising the r eac

T a b l e 4. I n v e s t m e n t a n d P r o d u c t i o n C o s t s fo r Au t o t h e r m a l R e f o r m i n g

produ ction cost ) b(m 1

(S/C))(m 2

(H 2/CO)) (7)


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exit tempera tur es above 900 C. Temperat ures higherthan 900 C are not normally allowed in steam-CO 2reformers because they would cause a drastic reductionof reformer tubes l ifetime. Literatur e indication re-ported by Parks and Schilmoller (1996) suggest thatalloys which would allow higher reforming temperaturesalready exist, but to our knowledge such materials havenot yet been used in steam-CO 2 reformers.

High percentages of u nconverted methan e wouldincrease the costs of some of the main downstreamprocesses for syngas mixtures with H 2/CO ) 2 . As

already mentioned in the Introduction paragraph, t

discussion of th e int egration between syngas pr oductand utilization technologies is not the objective of twork; neverth eless, this issue will be furth er a ddresin the conclusion paragraph.

Au t o t h e r m a l R e f o rm i n g . Table 3 l ists the cstudied for the autothermal-reforming simplified procscheme. Again th e economic ana lysis has considecases with production capacities of 300 000 N m 3/h of+ CO mixtures with H 2/CO r a t ios in t h e r a nge of 1w hi l e t he S/ C and O2/C r a t ios in t he feed h a ve bevaried r espectively between 0.5-1.5 and 0.55-0.6. Tvalues of O2/C ) 0.6 v/v and S/C ) 1.5 v/v can considered as the lowest limits of current technologi

F i g u r e 5 . Normalized production costs of synth esis gas fromautothermal reforming vs S/C ratio in the feed. The points referto the data of Table 4 calculated assuming that makeup CO 2 isrecovered from flue gas. The lines result from the regression ofth e d a ta o f T a b le 4 c a lc u la te d a s s u m in g th a t m a k e u p CO 2 isavailable free. The parameters of eq 7 are (a) volumetric basissm 2) 0.72, m 1 ) 1.13, b ) 7.73; (b) weight basissm 2 ) 1.08, m 1 )

1.16, b ) 79.29.

T a b l e 5 . C o m b i n e d R e f o r m i n g : C a s e s S t u d i e d ( C a p a c i t y o f 3 00 00 0 N m3/h o f a n H 2 + C O M i x t u r e )

F i g u r e 6 . Normalized production costs of synthesis from cbined reforming vs S/C ratio in the feed. The points refer todata of Table 6 calculated assuming tha t ma keup CO2 is r ecovefrom flue gas. The lines result from the regression of the datTable 6 calculated assuming that makeup CO 2 is available fThe par amet ers of eq 7 are (a) volumet ric basissm 2 ) 0.98, m1.28, b ) 3.70; (b) weight basissm 2 ) 1.30, m 1 ) 1.27, b ) 44


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The results of the sensit ivity an alysis are reportedin Table 4. Figure 5a,b shows th e variations of norma l-ized production costs vs S/C ratio in the feed. Costana lysis has been ma de by considering the m ake-up CO2as available free in the plant (lines in Figure 5) or byincluding t he recovery costs of make-up CO 2 from theflue gas (points in F igure 5). The resu lts of Table 4 showthat only cases AR/A, AR/B, and AR/F need to importextra CO2, in relatively low amounts, to achieve H 2/COratios between 1 and 1.5 v/v.

As for steam-CO 2 reforming, investment an d produc-tion costs decrease significantly when the S/C is re-duced, keeping consta nt O2/C a n d H 2/CO r a t ios. In t h ecase of synthesis gas with H 2/CO ) 2 the investmentan d production costs a re redu ced respectively by 18 and13%, lowering t he S /C from 1.5 to 0.6 an d t he O 2/C from0.6 to 0.55 v/v.

C o m bi n e d R e f o rm i n g . T abl e 5 l is t s t he cas esstudied for the combined-reforming process scheme: twodifferent rat ios of S/C in the feed ha ve been evaluat ed(2.5 and 0.5) to produce syngas at ratios of H 2/CO int he r ange 1-3 . T h e O2/C r a t io in t h e feed is a ss u m edequal to 0.5 v/v. The results of cost estimate a na lysisare reported in Table 6.

I t can be pointed out that cost variations are morerelevant than those in the other reforming technologies.The syngas with H 2/CO ) 2 v/v has fixed investmentcosts which ra nge from 291.5 to 186 MMUS$ (36% less)if the S/C varies from 2.5 to 0.5, while production costs

are reduced from 5.1 to 3.8 c/Nm 3 (26% less) withneed of make-up CO2.

Figure 6a ,b gives the t rend s of norma lized productcosts vs S/C ratio in the feed for different H 2/CO r atin the product on a volumetric and weight basis. Tpoints in Figure 6 are relative to the data of Tablereferring to the alternative of CO 2 recovered from fgas. The l ines in Figure 6 ha ve been obtained witregression of data presented in Table 5, referring t o alternative of CO2 available free in the plant.

Co m p a ri s o n a m o n g R e fo r m in g Te c h n o l ogC o s t s . I n T abl es 7 and 8 ar e s um m ar i zed t he m aoperat ing paramet ers tha t we assume, for the purpo

T a b l e 6. I n v e s t m e n t a n d P r o d u c t i o n C o s t s fo r Co m b i n e d R e f o r m i n g

T a bl e 7 . Ma i n O p e r a t in g P a r a m e t e r s o f P r o v e nT e c h n o l o g i e s

S /C O2/CP

(kg/cm 2)T out 1reformer

T out 2reformer

CH 4% resi(dry gas ba

SR 3 28 850 3-4AR 1.5 0.6 30 800-900 2CR 2.5 0.5 30 700 1100 0.1-2

T a b l e 8. M a i n Op e r a t i n g P a r a m e t e r s o f I n n o v a t i v e

T e c h n o l o g i e s ( L o w S / C a n d O2/C)

S /C O2/CP

(kg/cm 2)T out 1reformer

T out 2reformer

CH 4% resi(dry gas ba

SR 1 28 850-900 6-14AR 0.5 0.6 30 880-990 2CR 0.5 0.5 30 700 1100 0.1


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of the present work, a s r epresentat ive pa rameters forproven and for innovative technological solutions.

The estimated costs, for the capacity of 300 000 Nm 3/hof H 2 + CO gas mixture, have been sum mar ized in Table9. In order to compare t he different technologies on thes am e bas is , addit i onal cos t s for m ake-up C O2 a r eaccounted for in all cases in the darkened areas.

H2/ CO ) 1 v/ v Cases. Autothermal reforming hasthe smallest investment costs both for innovative andproven t echnologies: even if its oxygen dem and , andtherefore the cost of the air separation unit (included

in th e ISBL) is higher th an th e combined reforming one,it can still enjoy the benefits of a cheap reactor systemand of a s m al l er C O2 removal section. Autothermalreform ing also ha s th e lower pr oduction costs if th e S/Cand O2/C va lu es ar e m a in t ain ed in t he r a nge of pr oventechnologies. However, combined reforming benefitsmore th an an y other technology from the possibility ofreducing S/C and O2/C r at ios an d h as t he lower pr odu c-tion costs in the innovative conditions.

H2/ CO ) 2 v/ v Cases. In the conditions presentlyadopted by proven technologies, steam-CO 2 reformingand aut othermal reforming ha ve lower an d compar ableinvestment costs while combined reformer results in amore capital intensive technology. However t he lowconsumption and the higher thermal efficiency of com-

bined reforming allow one to reach the lowest syngasproduction costs even at S/C and O2/C va lu es a lr ea dyavailable in proven technologies. The advantages of combined reforming over steam-CO 2 reforming andautothermal reforming become much more evident if S/ C and O2/C va lu es in t he feeds a r e r ed u ced a t t h evalues we defined as innovative. In th is case combinedreforming would benefit from lower investments andmuch lower production costs t han the other syngasproduction techn ologies.

H2/ CO ) 3 v/ v Cases. Table 9 shows that also atH 2/CO ) 3 v/v both investment and production costs ofs t eam C O2 reforming are lowered by decreasing theamount of steam introduced in the feeds. However,

according to the equilibria syngas production reactions(3)-(6) the reduction of S/C ratios below the l imitschar acter istic of proven tech nologies (see Table 8) wouldnot allow one to produce a syngas with H 2/CO ) 3 v/vat the exit of combined- an d autothermal-reformingrea ctors. These last technologies would dema nd a shiftconverter to increase the H 2/CO r at io. Th is st ep wou ldnot n ecessarily cause a dramat ic increase of syngasproduction costs; h owever, th e discussion of this itemis beyond the limits of the present work.

C o n c l u s i o n s

The comparative economical analysis has shown thatsyngas investment and production costs are strongly

affected by S/C and/or O2/C va lu es . If t h es e r a t ios varied below the limits currently feasible, the syngcosts would be strongly reduced particularly when lH 2/CO r a t ios a r e n eede d. Th e r ed u ct ion of su ch r a twould be allowed by innovative catalysts particulainactive toward carbon format ion reactions (noble mebased catalysts) or, in the steam-CO 2-reforming conly, by process conditions in which carbon formaton conventional Ni-based catalysts, is inhibited wsmall am ounts of sulfur compounds in t he feedstoc

The costs of the three different technologies, ba

on a capacity of 300 000 Nm3

/year a n d a ss um in g t hno CO2 is present in the natural gas, have also becompared showing that combined reforming allows strongest relative reductions of t he production ainvestment costs by lowering the S/C and O2/C. Cobined reforming is also by far th e inn ovat ive technolmore favored for a syngas production in which H 2/) 2 v/v. Autothermal is less capital intensive thcombined reforming for the production of H 2/CO ) 1 vhowever combined reforming still maintains the lowproduction costs. Steam-CO2-reforming technology the highest variable costs, particularly at low H 2/ratios; these costs are related to the high fuel consumtion in the reformer furnace and to the strong need CO 2 import, which implies h igh consum ption of ste

for turbines and of cooling water in the CO 2 remosection.

In many cases the possible cost reductions have beestimat ed ar ound 25%; we consider t ha t su ch reductistrongly motivate innovative solutions concerning ptial filling of the r eformer tubes with innovative n ometal based catalysts or concerning sulfur passivareform ing. These techn ologies could also be appliedexploiting economically gas fields where t he n at ur al has a hi gh cont ent i n C O2 (>10%), as i t happensIndonesia or in some Saudi Arabia locations.

The impact of S/C and O 2/C r ed u ct ion s on t h e enom i cs of t he dow ns t r eam pr oces ses has not bincluded in this study. Indeed the specific requiremeof each downstream process in t erms of syngas quawould have resulted in a considerably broader and mcomplex ana lysis. Nevertheless, some genera l inditions can be drawn.

Presented da ta indicate that the economic advantwould be part icularly significant for those conversprocesses demanding a CO-rich syngas, such as osynthesis conversion, iron ore r eduction, acetic asynthesis, and th e hydroformylation process. The Fcher-Tropsch process, requiring an H 2/CO ratio of 2 aoperating a t a relatively low pressure (ca. 2-3 Mwith low limitation on CH 4 and CO2 conten t, should atake a dvantage of S/C and O2/C r ed uct ion .

Syngas production processes operat ing at high prs ur e (>5 MPa) and integrated with syngas conversi

T a b l e 9 . C o m p a r i s o n o f In v e s t m e n t a n d P r o d u c t i o n C o s t s f o r P r o v e n a n d I n n o v a t i v e T e c h n o l o g i e s ( L o w S / C a n d O2/C


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processes with str ingent l imitations on CH4 residue(such as th e NH 3 processes) would be instea d pen alizedby the reduction of S/C an d O 2/C r at ios a t const a nt H 2/CO, since high un converted m etha ne percenta ges wouldsignifican tly increase compression and recycle costs.Alternatively higher temperatures than those reachedin conventional tu bular reactors for st eam-CO2 reform-ing would be necessary to reduce the meth an e residue.

Methan ol production processes, with required rat iosin th e feed (H 2-CO 2)/(CO + CO 2) and CO2/CO r es pec-tively higher than 1.9 and 0.1, would need a case bycase study in order to evaluate the impact of the S/Ca n d O2/C r ed u ct ion s on t he over a ll econ om y, m ain lybecause many alternative process schemes and tech-nologies ar e possible. For MeOH plants with a capa cityof 3000 MTPD which demand a syngas feedstock of 300 000 Nm 3/yea r , t h e ut iliza t ion of a low S/C , O2/Ccombined-reforming technogy would be part icularlyconvenient because it would allow one to operate theprimary reformer at relatively low exit temperatures,leaving relatively high CH 4 residue in t he exit gas whichcould be reduced in the secondary r eforming.

L i t e r a t u r e C i t e d

Basini, L.; Sanfilippo, D. Molecular Aspects in Syngas Produc-tion: the Steam CO2 Reforming Reaction Case. J. C a ta l. 1995,157, 62.

Ch ou d a r y , V . R.; M a m m a n , A. S .; S a n s a r e , S . D . S e le ctiveOxidat ion of Metha ne t o Carbon Monoxide and Hydrogen overNi/MgO a t Low Temperatures. An g ew . C h em. I n t. Ed . En g l .1992, 9, 31.

Dave, N.; Foulds, G. A. Comparat ive Assessment of Cat alyticPart ial Oxidation and Steam Refoming for the Production of Methanol from Natural Gas. I n d . En g . C h em. Res. 1995, 34 ,1037.

Dibbern, H . C.; Olesen, P.; Rostru p-Nielsen, J . R.; Tott rup, P . B.;U d en g a a r d , N . R. M a k e L ow H 2/CO Syn ga s Us in g Su lfu rPassivated Reforming. Hyd rocarbon P rocess. 1986, Jan uary, 72.

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Gadalla, A. M.; Sommer , M. E. Synth esis and Ch ara cterizatioCa ta ly s ts in th e S y s te m A l2O3-MgO-Ni O-Ni for MethReforming with CO2. J. Am . C era m. S o c. 1989, 72 (4), 683

Grada ssi, M. J.; Green, N. W. Economics of Natu ral Ga s Consion Processes. Fuel Process. Technol. 1995, 42 , 65.

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Lange, L. P.; Tijm, P. J. A. Processes for Converting Methan

Liquid F uels: Economic Screening th rough Energy Manam e n t. C h em. En g . S ci. 1996, 51 , 2379.Par ks, S. B.; Schillmoller, C. M. Use Alloys t o Improve Et hyl

Production. Hyd rocarbon Process. 1996, March 56.Richar dson, J. T.; Par ipatyada r, S. A. Carbon Dioxide Reform

of Methane with Supported Rhodium. Appl. Catal. 1990,293.

Rostrup-Nielsen, J. R. Catalytic Steam Reforming. In CatalScience and Technology; Anderson, J. R, Boudardt, M., ESprin ger-Verlag: Berlin /New York/Tokyo, 1984a; Vol. 5.

Rostrup-Nielsen, J. R. Sulfur-Passivated Nickel CatalystsCarbon-Free Steam Reforming. J. C a tal. 1984b, 85 , 31.

Rostrup-Nielsen, J. R. Pr oduction of Synth esis Gas. Catal. To1993, 18, 305.

Ros tr u p -N iels en , J . R.; Ba k -H a n s e n , J . H . CO2-ReformingMethane over Transition Metals. J. C a ta l. 1993, 144 , 38.

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Sulfur Passivated Reforming Process Lowers Syngas H 2Ratio. Oil Ga s J. 1992, March 9, 62.

R eceiv ed for rev iew May 30, 1R evi sed m an u scr ip t recei ved October 9, 1

A ccept ed October 10, 19

IE97040

X Abstra ct published in Advance ACS Abstracts, Decem1, 1997.


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