1 Routes to Syngas. 1.1 General trends Towards focus and sustainability

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1 1 Rutes t Syngas 1.1 General trends Twards fcus and sustainability Synthesis gas (syngas) is a mixture f hydrgen, carbn mnxide and carbn dixide. It may als cntain nitrgen as applied fr the ammnia synthesis. Syngas is a key intermediate in the chemical industry. It is used in a number f highly selective syntheses f a wide range f chemicals and fuels, and as a surce f pure hydrgen and carbn mnxide. Syngas is playing an increasing rle in energy cnversin [418]. Synthesis gas can be prduced frm almst any carbn surce ranging frm natural gas and il prducts t cal and bimass by xidatin with steam and xygen. Hence it represents a key fr creating flexibility fr the chemical industry and fr the manufacture f synthetic fuels (synfuels). Figure 1.1 Cnversin via syngas. The cnversin via syngas results in prducts plus heat (Figure 1.1). In mst plants, the heat is utilised fr running the plant. As an alternative, the heat may be exprted, but that is nt always necessary. The present use f syngas is primarily fr the manufacture f ammnia (in 006, 14 millin tnnes per year) and f methanl (in 005, 33 millin tnnes per year), fllwed by the use f pure hydrgen fr hydrtreating in refineries as shwn in Table 1.1. The main cmmdity prducts based n natural gas are shwn in Table 1.1 [40]. It is evident that the chemical cnversin f natural gas 3

2 4 Cncepts in Syngas Manufacture (apprximately GJ/y) is marginal t the ttal natural gas prductin ( Nm 3 /y [78] r GJ/y assuming a lwer heating value (LHV) equal t 38 MJ/Nm 3 ). Recent trends in the use f syngas are dminated by the cnversin f inexpensive remte natural gas int liquid fuels ( gas t liquids r GTL ) and by a pssible rle in a future hydrgen ecnmy mainly assciated with the use f fuel cells. These trends imply, n the ne hand, the scale-up t large-scale GTL plants (mre than 500,000 Nm 3 syngas/h) and, n the ther hand, the scaledwn t small, cmpact syngas units fr fuel cells (5 100 Nm 3 syngas r H /h). These frecasts create new challenges fr the technlgy and fr the catalysis. Prduct Ammnia 14 Ethylene 75 Prpylene 53 Methanl 3 Hydrgen 0 Synfuels 18 d Yearly prd. (mil. t/y) Table 1.1 Main chemical prducts based n natural gas. Energy cnsumpt. (GJ/t) 9 15 b Thermal LHV Practical (%) c 60 Efficiency Ideal (%) CO (t/t) 16 a a) incl. CO cnverted int urea b) data kindly prvided by F. Dautzenberg, ABB Lummus, 005 c) CH 4 used fr reactin heat; n steam exprt d) excl. 3 millin tnnes per year under cnstructin Main technlgy Syngas/synthesis Steam cracking C H 6 Steam cracking C H 6 Syngas/synthesis Steam refrming Syngas/synthesis The data in Table 1.1 shw that the practical efficiencies fr natural gas cnversin int prducts are apprximately 80% f the ideal values expressed as: LHV prduct / ml ideal (1.1) LHV methane/ ml Fr endthermic reactins (ethylene, hydrgen), the LHV f the fuel prviding the reactin heat shuld be added t the nminatr. The wrld energy prductin is dminated by fssil fuels as the main energy surce. It amunted t 88% in 007 with il respnsible fr 37%

3 Rutes t Syngas 5 [78]. The energy cnsumptin is grwing fast in Asia, and China has becme the wrld s secnd largest cnsumer f il, after the USA. The prved reserves f il are cncentrated in the Middle East (61%) and thse f natural gas are als in the Middle East (41%), fllwed by Russia (3%) [78]. Cal is mre evenly distributed between the cntinents. Apart frm the large reserves (Middle East, Russia), natural gas is present as assciated gas in il fields. Hwever, as many fields are far frm the marketplace and ften ff-shre, the gas there is called remte gas r stranded gas [10]. Part f the assciated gas is reinjected t enhance the il recvery, but unfrtunately still a significant fractin is flared fr cnvenience. The flared gas amunts t clse t 5% f the ttal natural gas prductin (crrespnding t abut 1% f ttal wrld CO prductin frm fssil fuels) [63] [40]. S far, the prven reserves fr il have fllwed the increase in prductin as expressed by the reserves/prductin rati (R/P rati) staying at abut 40 fr il ver the last 0 years; hwever, at a steadily increasing cst f explratin and prductin. A big fractin f the reserves is present as il sand (tar sand) and ther nn-cnventinal surces under active develpment [78]. This means that at the present wrld prductin, the il reserves knwn tday will be used up within abut 40 years. This figure shuld be cnsidered with care. It des nt include reserves still t be fund and it des nt include the changes in cnsumptin (fr instance the grwth in Asia). Furthermre, the R/P rati fr il varies frm regin t regin, being abve 80 in the Middle East and belw 0 in Nrth America. The R/P rati (007) fr natural gas is abut 60 and 1 fr cal [78]. The ttal R/P fr fssil fuels (based n il equivalent) is less than 100 years. These figures emphasise the need fr flexibility in the energy netwrk and the need fr alternative fuels. Oil is the mst versatile f the fssil fuels with high energy density and it is easily transprted. The pwer industry is very flexible t feedstcks and it is feasible t transprt cal ver lng distances t big centralised pwer plants clse t deep water harburs. Natural gas is transprted t the marketplace in pipelines ver still lnger distances r as liquified natural gas (LNG).

4 6 Cncepts in Syngas Manufacture The autmtive sectr represents a special challenge as the energy cnversin is strngly decentralised. S far il-derived prducts have been the slutin, but in view f the limited reserves f il, a number f alternative fuels are being cnsidered, such as liquefied petrleum gas (LPG), natural gas, methanl, dimethylether (DME), ethanl, bidiesel, synfuels and hydrgen. Bifuels represent a sustainable respnse t liquid fuels. It may be based n ethanl and bidiesel derived frm cnventinal agricultural prducts r frm synfuels via gasificatin f bimass. The alternative fuels may be blended with cnventinal fuels r used directly in internal cmbustin engines (ICE) r fuel cells. In Western Eurpe alternative fuels may amunt t 0% f energy surces by 00. Glbalisatin has caused cmpanies t cncentrate n cre business and critical mass. It has resulted in a restructure f the chemical industry int tw types f fcused cmpanies [190]: the mlecule suppliers (cmmdities and fine chemicals) and the prblem slvers (functinal chemicals like additives and pharmaceuticals). Each type has its wn characteristics as reflected by the rle f the catalyst [418]. The mst imprtant parameter fr large-vlume chemicals is prductin csts (variable and fixed csts). The variable csts are related t the feed csts, the use f energy, prcess selectivity and envirnmental csts. Fur trends have characterised plants fr cmmdity chemicals: Lcatin f cheap raw materials; Ecnmy f scale; Mre integrated plants; and CO ftprint (tnnes CO per tnne prduct). Plants are mved t lcatins where raw materials are cheap. As illustrated in Figure 1., the ammnia prductin is hardly feasible at natural gas prices typical fr Eurpe and USA (3 4 USD/GJ with high seasnal variatins) [40]. As a result, new plants fr cmmdity chemicals are built at lcatins (Middle East, Trinidad, Nigeria, West Australia ) with lw natural gas prices (0.5 1 USD/GJ). It means that

5 Rutes t Syngas 7 the use f natural gas as feedstck may nt be feasible where there is a big market fr natural gas as fuel. Figure 1. Ammnia prductin csts [40]. Reprduced with the permissin f Springer. Plants have becme larger t take advantage f the ecnmy f scale. The ecnmy f scale can be expressed by: capacity1 Cst1 Cst capacity (1.) n typically varies between The ecnmy f scale means chice f different technlgies as they may be characterised by different values f n. Tday, ammnia plants are built with capacities up t mre than 3000 metric tns per day (MTPD) and methanl plants are being cnsidered at capacities f MTPD. This crrespnds t the size f synthetic fuel plants based n FT synthesis (35,000 bpd). At the same time, as plants becme bigger, there is a trend t minituarise chemical prcess plants and take advantage f mass prductin, the ecnmy f numbers cmpeting with the ecnmy f scale. This is ne f the key issues in the hydrgen ecnmy and the applicatin f fuel cells. Micr-structured n

6 8 Cncepts in Syngas Manufacture prcess equipment cmpnents such as heat exchangers, and new reactr cncepts are becming available. Plants have als becme mre integrated t minimise energy cnsumptin. It can be shwn that the plant csts fr a variety f prcesses crrelate with the energy transfer (heat transfer, cmpressin) within the prcess scheme [89]. As an example, the energy cnsumptin f ammnia prductin has decreased ver the last 50 years frm abut 40 GJ/t t 9 GJ/t crrespnding t a thermal efficiency (LHV) f 65% r 73% f the theretical minimum [169] [40]. Cmmdity plants depend n steady imprvement and sphisticatin f the technlgy. Even small imprvements f the prcess scheme may shw shrt payback times. On the ther hand, the uncertainties assciated with new technlgy may easily utbalance the ecnmic advantage f a new prcess. Imprvement f ne prcess step might easily result in less favurable perfrmance f anther prcess step. The high degree f integratin means that the weakest part f the chain may determine the perfrmance f the entire plant. As an example, there is a need fr mre cke-resistant catalysts and ften deactivatin phenmena determine the prcess layut and the ptimum prcess cnditins t be applied [404]. It is evident that catalyst life, i.e. n-stream factr, is crucial fr large-scale cmmdity plants in cntrast t batch-wise manufacture f fine chemicals. A few days prductin stp because f a catalyst failure may be crucial fr the plant ecnmy. It means that secndary phenmena such as catalyst deactivatin are imprtant issues. Fr large-scale peratin, ecnmic arguments will limit the minimum space time yield t apprximately 0.1 tnne prduct/m 3 at a typical catalyst life f 5 years [89]. This crrespnds t a catalyst cnsumptin f less than 0. kg cat/t prduct. Fr ammnia synthesis a typical figure is 0.03 kg cat/t NH 3. These risks mean that it has becme mre expensive t develp new prcess technlgy. New technlgy must be demnstrated t a larger extent nt nly the basic principles, but als the slutins t a series f secndary prblems [400] [418]. Many well-established prcesses are appraching their theretically achievable efficiency, selectivity, etc. (refer t Table 1.1), but new

7 Rutes t Syngas 9 challenges have been intrduced by bjectives fr sustainable grwth frmulated by sciety. This has nt nly led t the intrductin f new prducts, but als necessitated the develpment f new prcesses. Envirnmental challenges represent majr rm fr breakthrughs in the catalytic prcess industry. Fr any prcess scheme, it is essential at an early stage t establish the verall mass balance and t estimate the ΔP as simply being the difference between the price f prducts and the price f feedstcks [418]. Hence, there has been a trend t develp prcesses using cheaper raw materials. The gain in ΔP culd, hwever, be lst by lwer selectivity r higher investments. Selectivity is crucial t achieving a high ΔP. Lw selectivity and cnversin per pass result in lw cncentratins in prcess streams and hence mre expensive separatin systems. Figure 1.3 Simplified mass balance. It may be argued that energy efficiency is f less imprtance when natural gas is cheap, but high energy efficiency means small feed pretreat units and reduced requirements fr utilities and hence less investments. Mrever, high efficiency means less CO prductin. As illustrated in Figure 1.3, the ΔP calculatin shuld cnsider als the energy

8 10 Cncepts in Syngas Manufacture cnsumptin and the by-prducts which may easily have a negative value. This may be expressed by the s-called E-factr [458] expressing the amunt f by-prduct prduced per kg f prduct. The emissins may have great negative value. This may be reflected by the csts f carbn capture and strage (CCS) (refer t Sectin 1.4.). The CO emissin expressed as a C-factr [106] [40] (tnnes CO per tnne prduct, refer t Table 1.1) may becme an imprtant prcess parameter in the future. CO emissins are ften directly related t the energy cnsumptin f the prcess. As an example, a reductin f the energy required t prduce ammnia frm natural gas f 1 GJ/t means a reductin f CO emissins f arund 8.5 millin tnnes CO /y, wrldwide [416]. In many ammnia plants abut 80% f the CO is reacted with ammnia t urea frm which it is, f curse, liberated t the atmsphere when the urea is used as fertiliser. On the ther hand, CO -cnsuming prcesses will hardly change the picture frm nn-carbn cntaining fuel [416]. As an example, cnsider the methanl synthesis: CO 3H CH3OH HO (1.3) Even if hydrgen was made available frm nn-carbn cntaining fuel, the present wrld prductin f methanl via this reactin wuld nly amunt t 40 millin t CO /y. This crrespnds t the CO emissin frm a 4000 MW cal-based pwer plant and shuld be cmpared with the ttal CO emissins f apprximately t CO /y ( t carbn/y). It means that CO as a reactant will have little impact n the CO prblem. Again, the prducts will eventually end as CO. A similar argument is valid fr CO refrming f natural gas Direct r indirect cnversin An imprtant challenge in C 1 chemistry is t circumvent the syngas step by a direct cnversin f methane int useful prducts. Still, yields are far frm being ecnmical [38] [87] [307]. The methane mlecule is very stable, with a C-H bnd energy f 439 kj/ml; hence methane is resistant t many reactants. Electrphillic attack requires superacidic

9 Rutes t Syngas 11 cnditins, and radical abstractin f a hydrgen atm by a reactant Q requires that the Q-H bnd exceeds 439 kj/ml [130]: Q CH QH (1.4) CH 4 3 This is feasible when Q is an xidising agent. Hwever, the prduct ften has much weaker C-H bnds than methane, which implies that it is difficult t eliminate further reactins leading t cmplete xidatin. O O CH 4 CH xo CO H O (1.5) The direct cnversin f CH 4 int methanl may have a high selectivity, but at a lw cnversin per pass. Fr example, Zhang et al. [537] reprted a selectivity f 60% at a cnversin f 1 13%. This crrespnds t a yield f abut 7.5%. This lw yield per pass results in a large recycle rati and a difficult separatin assciated with a lw partial pressure f the prduct. This is illustrated by simple calculatins in Figure 1.4 [410]. Figure 1.4 Recycle rati and cnversin. A selectivity f 95% at a cnversin per pass f 5% means a large recycle rati f apprximately 10, and hence a difficult separatin due t lw partial pressure f the prduct. Reprduced with the permissin f Springer [410].

10 1 Cncepts in Syngas Manufacture A simple kinetic analysis f cnsecutive first-rder reactins [410] may illustrate the prblem. The data in Figure 1.5 shw that the higher the rati k /k 1, the lwer the yield f the intermediate B. Figure 1.5 Cnsecutive reactins. Reactivities and maximum yields. Reprduced with the permissin f Springer [410]. The direct xidatin f methane t methanl r frmaldehyde has been a dream reactin fr a lng time [537]. Attempts include gasphase reactin, catalytic reactins, and use f ther xidants than air. Selectivities may be high, but at a lwer cnversin per pass resulting in yields being inferir fr industrial use. High selectivity and cnversin may nt be sufficient. A prcess using superacid activatin (Catalytica) fr cnverting CH 4 via methyl bisulphate int methanl has the ptential f achieving a high selectivity f 95% at a cnversin f 90% [345]. Hwever, the prcess wuld require a large sulphuric acid plant (1 ml SO 3 /ml methanl) and a unit fr cncentrating a large recycle f diluted acid [410]. Other attempts have aimed at creating a carbn-carbn bnd frm methane, althugh mst natural gas surces cntain a fractin f ethane and ther lwer alkanes.

11 Rutes t Syngas 13 Mst wrk in direct cnversin has fcused n the xidative cnversin f methane int ethylene [196] [307]. It has prven t be mre prmising than high-temperature pyrlysis f methane int primarily acetylene. Hwever, the prcess suffers frm ethane being a significant part f the prducts (lw ΔP) and that abve 0% f the cnverted methane is xidised t carbn xides. Under industrial cnditins C + yields are less than 0% at a cnversin f 4 35% per pass. As a result, the prcess scheme ends up being rather cmplex, meaning that the xidative cupling is nt ecnmically feasible with the present lw selectivities t C hydrcarbns. Althugh the reactin scheme is elegant, the principles behind Figure 1.4 may explain why yields in xidated cupling have never passed an apparent ceiling [9]. Catalytic partial xidatin at high temperature and ultra-shrt residence time ver nble metals gauze has shwn frmatin f lefins and xygenates [06]. The feasibility f this rute is still t be analysed. The indirect rute via methanl appears t be a mre prmising rute fr lefins (see Sectin.6). Direct cnversin f methane t higher hydrcarbns withut the assistance f xygen is nt favured by thermdynamics. This cnstraint can be circumvented in a tw-step prcess via carbides, but s far yields have been insignificant [79]. Other studies have explred the direct cnversin f methane int benzene [40]. Selectivities f 70% were btained clse t equilibrium cnversin at 600 C (1%). Frm a thermdynamic pint f view [145] the manufacture f synthetic transprtatin fuels shuld aim at a minimum change f the hydrgen cntent f the feedstck t that f the prduct (typically arund H/C=). It means that, in principle, it is mre efficient t cnvert natural gas t paraffinic diesel than t armatic rich gasline. Fr cal the indirect cnversin via syngas appears less efficient than the direct hydrgenatin rutes. Hwever, these theretical cnsideratins shuld be supplemented with an analysis f the prcess steps and selectivities invlved [145]. The main advantage f the indirect rutes via syngas is the very high carbn efficiency. As an example, a mdern methanl synthesis lp based n natural gas may perate with mre than 50% cnversin per

12 14 Cncepts in Syngas Manufacture pass having a selectivity f 99.9% and a carbn efficiency abve 95% (refer t Sectin.6.). The synthesis gas rutes are highly efficient as illustrated in Table 1.1, but they are capital intensive because they invlve exchange f energy in the refrmers and heat recvery units, as illustrated in Figure 1.6 [413]. Figure 1.6 Indirect cnversin f natural gas (numbers indicate the relative investments) [413]. Reprduced with the permissin f Elsevier. Syngas manufacture may be respnsible fr apprximately 60% f the investments f large-scale gas cnversin plants based n natural gas. Therefre, there is great interest in ptimising prcess schemes based n steam refrming and autthermal refrming as well as in explring new rutes fr the syngas manufacture. 1. Manufacture by steam refrming f hydrcarbns 1..1 Reactins and thermdynamics Steam refrming is the reactin between steam and hydrcarbns int a mixture f hydrgen, carbn mnxide, carbn dixide and uncnverted

13 Rutes t Syngas 15 reactants. Steam may be replaced by carbn dixide as reactant. The refrming reactins are accmpanied by the water-gas-shift reactin. The term steam refrming shuld nt be cnfused with catalytic refrming used fr the cnversin f paraffinic hydrcarbns t high ctane hydrcarbns such as is-alkanes and armatics. A better term may be xygenlysis [381] [389] as the reactin invlves the breakage f C-H and C-C bnds by means f xygen cntaining species. A cmplete steam refrming reactin scheme can thus be written as: Table 1. Steam refrming reactins. Reactin H 0 98 kj ml R1 CH4 HO CO 3H -06 R CH4 HO CO 4H -165 R3 CH4 CO CO H -47 R4 CO HO CO H 41 R5 CnHm nho nco (n 0.5m)H <0 Althugh all reactins may describe specific perating cnditins, nly tw ut f the first fur reactins are independent frm a thermdynamic pint f view, since the ther tw can be established as linear cmbinatins f the tw selected nes. Catalytic studies indicate that it is steam refrming f methane t carbn mnxide and the watergas-shift reactins that are the independent reactins in additin t the steam refrming f higher hydrcarbns as the last reactin. This set f reactins (R1, R4, and R5 in Table 1.) will cnsequently be used in the fllwing. The term steam refrming is als used fr the reactin between steam and alchls (methanl and ethanl) as well as liquid-phase reactins with carbhydrates r bimass (see Sectin 1.4).

14 16 Cncepts in Syngas Manufacture The last reactin R5 in Table 1. is the reverse Fischer Trpsch synthesis, but the cnversin f higher hydrcarbns can be cnsidered irreversible at nrmal refrming temperatures. The higher hydrcarbns react n the metal surface t C 1 cmpnents r stay as carbnaceus depsits. At temperatures abve C, the catalytic reactins may be accmpanied by thermal cracking. The symbl indicates that a reactin is reversible, i.e. at a given temperature the reactin will nt have full cnversin. Usually nly the last reactin is cnsidered irreversible, s parts f methane and steam will remain in the mixture at utlet cnditins. Preparatin f heat and mass balances fr synthesis gas prcesses thus requires methds t calculate mass and heat balances and chemical equilibrium. Fr a reactin system like the ne shwn abve in Table 1. it is cnvenient t define the thermdynamic reference state as the enthalpy and free energy f frmatins as an ideal gas at 5ºC (98.15 K). This definitin allws direct calculatin f heat duty in the enthalpy calculatin withut having t distinguish between the parts fr heating and reactin. Different functins may be used t represent the ideal gas heat capacity r enthalpy as a functin f temperature; values in Appendix 1 are based n a furth-degree plynmial fr the enthalpy f frmatin, where the zer-rder cefficient has been adjusted t btain the enthalpy f frmatin at 5ºC. k1 H i Ek T (1.6) 5 k1 Appendix 1 shws the cefficients in the enthalpy plynmials fr a number f characteristic synthesis gas reactin key cmpnents and a table with actual values f the ideal enthalpies f frmatin as a functin f temperature. The preparatin f mass balances requires calculatin f chemical equilibrium by slutin f the fllwing Equatin (1.7) fr the cupled reactins between all cmpnents: i1 i, j i K a (1.7) eq, j

15 Rutes t Syngas 17 where i,j is the stichimetric cefficient fr cmpnent i in reactin j. The right-hand side includes the activity, which rigrusly is defined as: a i gas fi f i yi i P fi (1.8) in which f is the fugacity, f the reference fugacity and φ the fugacity cefficient, which describes the deviatin frm an ideal gas. Preparatin f syngas is carried ut at high temperature and a mdest pressure s the assumptin f an ideal gas is acceptable when the peratin is nt clse t the dew pint f the mixture. This implies that the fugacity cefficient φ usually is set t unity. The equilibrium cnstant, K eq, is a functin f temperature nly, s the cnversin in the steam refrming f methane reactin is favured by a lw pressure. As the reference state fr all cmpnents in the reactins is defined as an ideal gas at 5ºC (98.15K) and bar, the value that must be used fr f is bar. If anther unit f measurement fr pressure is used, the reference pressure must be changed crrespndingly s that the activity becmes independent f pressure. If a cmpnent is a slid, such as carbn in carbn frmatin, the activity, a, is unity, since the reference state fr the Gibbs free energy f carbn is als defined as the slid state. The temperature equatin fr the equilibrium cnstant is derived frm thermdynamics using the Gibbs energy f frmatin, G, the enthalpy f frmatin H, and the temperature dependence as derived frm: RTln(K dln(k dt eq,j eq,j ) ) ΔH RT ΔG j j (1.9) After insertin f Equatin (1.6) the final equatin fr the equilibrium cnstant is: C, j 3 ln(keq, j) C1, j ln(t) C3, j C4, j T C5, j T C6,j T (1.10) T The basic reactin prperties fr sme key reactins are shwn in Table 1.3.

16 18 Cncepts in Syngas Manufacture Table 1.3 Basic reactin prperties fr steam refrming reactins. Frmatin data at 5ºC. Data frm [137] [375]. CH H O 4 CO 3H CO H O CO H CH CO 4 CO H CH HO 6 CO 5H H ihi i1 kj / ml 0 igi i G kj / ml 0 H G S T kj / ml / K The definitin f the standard state as the ideal gas f frmatin at 5ºC (98.15K) and bar implies that the abslute values f entrpy will nt fulfil the third law. The heat f reactin is defined as the heat f frmatin, but with the ppsite sign s it is written as - ΔH 98. Tables with equilibrium cnstants at selected temperatures fr the main synthesis gas reactins are shwn in Appendix. The refrming reactin invlving tw stable mlecules as methane and water is strngly endthermic and it leads t frmatin f mre mlecules. This means that the affinity fr the reactin (-ΔG ) is established by the entrpy term. The basic thermdynamic reactin prperties as a functin f temperature are shwn fr the methane refrming reactin in Figure 1.7. It is seen that reactin enthalpy and entrpy are weak functins f temperature, but als that they are psitive in the entire interval implying that ther heat surces are necessary fr cnversin. The free energy decreases strngly with temperature due t the crrespnding increase in

17 Rutes t Syngas 19 TΔS giving mre favurably equilibrium. Hence, the steam refrming reactin is entrpy-driven. Figure 1.7 Steam refrming. Thermdynamic functins. Cnversin at equilibrium is calculated by slving the cupled set f equatins fr K eq fr the tw independent reactins R1 and R4 in Table 1.. A simplified methd t find the cnversins in the tw reactins is available as will be shwn belw, but a general methd which can slve any chemical equilibrium prblem is preferred. Fr this purpse tw methds may be used. The first is minimisatin f the Gibbs free energy [316], whereas the ther ne is the slutin fr cnversins [468]. The first ne may be attractive frm a theretical pint f view and it is readily cmbined with phase equilibrium, but the last ne is preferred in catalysis, since n cmbinatin f reactins may prceed in all cases. The set f equatins in Table 1. may be slved using the Newtn Raphsn methd with the cnversins as independent variables. Sme f the cmpnents (higher hydrcabns r xygen) may almst disappear in the final mixture s it is necessary t handle eliminatin f reactins with almst cmplete cnversin.

18 0 Cncepts in Syngas Manufacture Figure 1.8 Steam refrming and methane cnversin [415]. Reprduced with the permissin f Elsevier. Methane cnversins as a functin f temperature in the cmbined methane refrming and shift reactins are shwn in Figure 1.8. It is seen that cnversin is strngly favured by high temperature, lw pressure, and high steam-t-carbn rati. Example 1.1 A simplified methd [76] may be used t slve the chemical equilibrium fr the reactins R1, R4 and R5 in Table 1. and find the cnversins ξ 1 and ξ 4 in the reactins R1 and R4. Initially all higher hydrcarbns are cnverted quantitatively t CO and H using Equatin R5, whereafter the mle amunts in the initial

19 Rutes t Syngas 1 mixture are cnverted accrding t the rdinary stichimetric cnversin equatins belw: In Out y y y y y y CH4 HO CO CO H y y y y y CH4 HO CO CO H y y y y y CH4 HO CO CO H (y (y (y (y (y CH4 HO CO CO H ) /(1 ) ) /(1 ) ) /(1 ) 1 ) /(1 ) ) /(1 ) The bttm line is the ttal amunt befre and after cnversin. The nrmalisatin is in principle with respect t the ttal amunt f precnversin f higher hydrcarbns; here it is simply assumed that the sum f the initial mle fractins is unity. Assuming ideal gas the mle fractins can nw be inserted in the tw equatins fr K eq : K K eq,ref eq,shf y y 3 H y y CH4 H CO y y y CO y HO CO P HO Cnsidering the shift reactin alne results in a secnd-rder equatin in the cnversin 4, and this equatin can be slved analytically after refrmulatin f the equilibrium equatin as: K eq,shf 1 4 K eq,shf (y y ) (y y ) CO HO H CO K eq,shf yco y y y 0 HO r A 4 B 4 C 0 H CO 4 The rts are : B 4 B 4AC B D A A

20 Cncepts in Syngas Manufacture Bth rts are real, but nly the smallest ne is used, since the largest ne (by experience) may give negative mle fractins. B is always psitive, s the slutin is in principle nly relevant fr D, but since B and D numerically may have the same size, it is apprpriate t use the + sign fr the ther rt and then calculate the final rt by using the fact that the prduct f the tw rts is C/A. The cnversin in the shift reactin alne is then: 4 C B D This implies that fr any value f 1 the cnversin in the shift reactin can be fund analytically. The refrming reactin with knwn 4 is a furth-rder equatin in 1. It can be slved analytically, but it is mre practical t implement a cmbined methd t slve the tw equatins [76]. This methd uses bisectin fr the determinatin f 1 with insertin f the crrect value f 4 fr each step by slutin f the quadratic equatin. The equatins are highly nn-linear, s it is necessary t knw the cnversin limits. The lwer limit may be set t zer here. It need nly be cnsidered if the reverse reactin must be cnsidered. The upper limit is the minimum cncentratin f either CH 4 r H O+CO. The methd is suitable fr implementatin in a spreadsheet and can be written as: 1. Calculate the equilibrium cnstants fr the tw reactins frm the data in Appendix.. Specify the lwer and upper bundaries as 1,min and 1.max. 3. Guess a new value f 1 as the average f the minimum and maximum values. 4. Calculate new initial mle fractins assuming the cnversin 4 in the shift reactin is Find the cnversin in the shift reactin frm the analytical slutin. 6. Calculate new mle fractins using bth cnversins. 7. Calculate the reactin qutient fr the steam refrming reactin by inserting the new calculated mle fractins in the equilibrium expressin. 8. If the reactin qutient is larger than the equilibrium cnstant, the cnversin is t large, since the reactin qutient increases

21 Rutes t Syngas 3 with cnversin. Set the upper cnversin limit t the calculated value f 1 and jump t step If the reactin qutient is smaller than the equilibrium cnstant, the cnversin is t small. Set the lwer cnversin limit t the calculated value f 1 and jump t step 3. The iteratin is slw but safe and cntinues until the desired accuracy has been reached. The steam refrming reactins are fast reactins due t the high temperature and the presence f the catalyst implying that the actual cnversin will be clse t the equilibrium cnversin. Figure 1.9 shws the cnversins in the methane steam refrming and shift reactins, if they are treated as independent reactins. The cnversin in the endthermic steam refrming is increasing with temperature, whereas the ppsite is the case fr the shift reactin. Figure 1.9 Temperature, cnversin and temperature apprach t equilibrium fr steam refrming and shift. Using actual cnversins as a measure f reactin extent is nt cnvenient, since they must be relative t equilibrium cnversins. Instead the s-called temperature apprach t equilibrium is used,

22 4 Cncepts in Syngas Manufacture which is the hrizntal distance between the actual utlet temperature and the crrespnding equilibrium temperature. This is illustrated in Figure 1.9 by use f the adiabatic reactin paths. The temperature apprach t equilibrium is prtrayed by hrizntal dtted lines. It is seen that an apprpriate definitin f the temperature apprach is: ΔTapp,ref Texit Teq (1.11) ΔT T T app,shf T eq is the temperature n the equilibrium curve having the same cnversin r the same reactin qutient as the actual utlet gas. By this definitin the temperature apprach will always be psitive prvided the reactin path is belw the equilibrium curves. It is true that if tw f the reactins in the reactin set R1 t R4 in Table 1. are at equilibrium, the ther tw will als be at equilibrium. If, hwever, ne f the reactins has an apprach larger than zer, the ther reactins will have different appraches and sme may even have negative appraches. This als signifies that cnversin and temperature apprach in fact nly has a practical meaning fr the limiting reactin. Example 1. The equilibrium cnversin in the steam refrming f methane t carbn mnxide and hydrgen is determined frm the tw equilibrum expressins shwn in Example 1.1. In this secnd example a large hydrgen plant making 100,000 Nm 3 /h f H is cnsidered. Althugh sme hydrgen must be present in the feed t keep the catalysts reduced, the feed is assumed t be pure CH 4 with an actual flw equal t 500 ml/s and a temperature f 500ºC. The cnditin ut f the tubular refrmer is 875ºC and 31 bar (abslute) and the dry gas cmpsitin (mle %) has been measured at the utlet as: H 70.36, CO CO 7.35 and CH Assume that the shift reactin is in equilibrium in rder t calculate the unknwn amunt f water and then calculate the temperature apprach t the steam refrming f methane reactin. The equilibrium cnstant fr the shift reactin at 875ºC is fund in Appendix t be The equatin fr the shift reactin can be rearranged as a functin f the dry mle fractins and the unknwn water mle fractin as: eq exit

23 Rutes t Syngas 5 K eq,shf y dry,h y y dry,co dry,co y (1 y H O H O Insertin f the specified dry mle fractins and the equilibrium cnstant gives: y mle% H O The wet mle fractins are nw calculated, and these and the pressure are then inserted int the steam refrming equilibrium reactin t calculate the reactin qutient. The result is: Q ref 544 T eq,ref 850 C T ) ref 5 C In additin t the steam refrming reactins carbn may als be frmed accrding t the reactins in Table 1.4. Table 1.4 Carbn-frming reactins. Reactin H 0 98 kj ml R6 CH4 C H -75 R7 CO C CO 17 R8 CO H C HO 131 R9 CnHm nc 0.5mH <0 Tw frms f carbn may be fund. The first is rdinary graphite, but carbn may als be fund in a whisker structure n the catalyst, which has anther free energy f frmatin as will be discussed in Chapter 5. The reactin prperties are seen in Table 1.5 belw. A table with equilibrium cnstants is fund in Appendix.

24 6 Cncepts in Syngas Manufacture Table 1.5 Basic reactin prperties fr carbn-frming reactins. Data frm [137] [375] fr carbn as graphite. Data frm [45] fr carbn as whisker (refer t Sectin 5.3.1). CH 4 C H CH C CO 4 whisker H C CO CO H C H O H kj / ml C ihi i i i i G G kj / ml 0 H G S T kj / ml / K (See ntes t Table 1.3) Prduct gas cmpsitin The methane steam refrming reactin R1 in Table 1. results in a H /CO rati clse t 3. Steam can be replaced by CO, resulting in a H /CO rati clse t 1. The additin f xygen in ATR and POX gives a lwer H /CO rati. The H /CO rati can be varied ver a wider range, as illustrated in Figure 1.10, as the refrming reactins are cupled t the shift reactin. In the manufacture f hydrgen, the refrming prcess is fllwed by water gas shift carried ut in the presence f an irn and/r cpper catalyst at lw temperatures ( C) t ensure cmplete cnversin f carbn mnxide (refer t Sectin 1.5). The cnversin f methane is restricted by the thermdynamics f the refrming reactins. The endthermic steam (and CO ) refrming reactins must be carried ut at high temperature and lw pressure t achieve maximum cnversin, as illustrated in Figure 1.8.

25 Rutes t Syngas 7 Figure 1.10 H /CO ratis frm varius syngas prcesses [415]. Reprduced with the permissin f Elsevier. SMR: steam methane refrming; ATR: autthermal refrming (see Sectin 1.3.); POX: partial xidatin (see Sectin 1.3.1). As an example, mdern hydrgen plants are nrmally designed fr lw steam-t-carbn ratis (refer t Sectin.), althugh high steam-tcarbn ratis (4 5 mlecules f H O/C atm) wuld result in higher cnversin f the hydrcarbns. Hwever, a lw steam-t-carbn rati (typically.5 r less) reduces the mass flw thrugh the plant, the steam prductin, and thus the equipment sizes. The lwest investment is therefre generally btained fr plants designed fr lw steam-t-carbn ratis. In principle, a lw steam-t-carbn rati increases the amunt f uncnverted methane frm the refrmer (Figure 1.8), but this can be cmpensated fr by increasing the refrmer utlet temperature, typically t 90 C. In synthesis plants, the uncnverted methane flws dwnstream with the synthesis gas. Uncnverted methane thus implies a larger syngas unit and results in restrictins n recycle ratis in the synthesis because f accumulatin f the inert methane in the synthesis gas, which reduces the partial pressures f the syngas cmpnents.

26 8 Cncepts in Syngas Manufacture Figure 1.11 Cmbined steam and CO refrming fr prducing a syngas with H /CO=. 5 bar abs. T exit =950 C [415]. Reprduced with the permissin f Elsevier. Stichimetric refrming accrding t Equatins R1 and R3 in Table 1. at H O/CH 4 r CO /CH 4 ratis f 1 is rarely feasible [399], because it wuld result in incmplete cnversin at the pressures that are ecnmical fr industrial syngas plants (0 50 bar). This is als true fr mixed CO /H O refrming as illustrated in Figure 1.11 [415]. The lwpressure manufacture f reducing gas fr direct re reductin is ne exceptin (refer t Sectin.4.3). Other thermdynamic cnstraints are related t the risk f carbn frmatin when t little xidant is present (refer t Chapter 5).

27 Rutes t Syngas 9 The requirements t the cmpsitin f the syngas vary with the synthesis in questin, as shwn in Table 1.6. Table 1.6 Syngas cmpsitin fr varius prcesses [415]. Prcess Stichimetric cmpsitin C-reactants Ammnia H M 3 N Methanl H CO M CO CO DME frm hydrcarbns H CO M CO CO DME frm cal gas H M 1 CO High-temp. Fischer Trpsch H CO M CO CO Lw-temp. Fischer Trpsch H M CO SNG H CO M 3 CO CO Acetic acid CO Methanl Higher alchls H Olefins M 1 CO Industrial hydrgen H Hydrgen fr PEMFC <50 ppm CO Reducing gas (irn re) CO HO 0.05 H COCO H O

28 30 Cncepts in Syngas Manufacture 1..3 Thermdynamics f higher hydrcarbns Higher hydrcarbns are steam refrmed accrding t Reactin R5 in Table 1.. In natural gas mixtures and light hydrcarbn mixtures the cmpnents are identified as such, but in heavier hydrcarbn feedstcks such as naphthas, kersene and diesel, the chemical cmpunds and cmpsitin are usually unknwn. Instead, such mixtures are characterised by the fllwing measurements: A distillatin curve with initial and final biling pint. It is cnverted t a true biling-pint (TBP) curve; Specific gravity; The carbn t hydrgen (C/H) weight rati; PNA distributin: Paraffin (alkane), Naphthene (cyclalkane), and Armatic; Lwer heating value (LHV); Refractive index; Liquid viscsity. Except fr the LHV, n reactin prperties are thus measured. Crrelatins are available t estimate reactin prperties using cnventinal refinery pseud-cmpnent characterisatin methds, but it is advantageus t calculate a mixture f real chemical cmpunds, which represent the measurements, as well as pssible t be able t calculate accurate mass and energy balances ver the plant. This is carried ut selecting a number f cmpnents, whse type is chsen apprximately prprtinal with the PNA distributin and the C/H rati. The cmpnents are selected in such a way that their nrmal biling pints are distributed equally n the measured TBP curve. Fr each additinal measurement an equatin is set up using prper mixing rules and the ttal equatin system is slved using apprpriate uncertainty factrs. After slutin f the equatins, the resulting cmpsitin f the mixture will give accurate mass and energy balances in the syngas prcess.

29 Rutes t Syngas The tubular refrmer The negative heat f reactin f the refrming reactin and the high exit temperatures at typical prcess cnditins mean that heat must be supplied t the prcess, typically in a fired reactr, the tubular refrmer [389] as shwn in Figure 1.1. The prduct gas will nrmally be clse t the equilibrium f Reactins R1 and R4 in Table 1. with an apprach t equilibrium f abut 5 10 C and 0 C fr the tw reactins, respectively. Figure 1.1 Pht f a tubular refrmer.

30 3 Cncepts in Syngas Manufacture T illustrate the strng endthermicity, adiabatic steam refrming f methane (carried ut with H O/CH 4 =.5 at a pressure f 0 bar abs and a feed temperature f 500 C) will result in a temperature drp f apprximately 1 C fr each 1% f methane cnverted. The verall heat requirement can be estimated frm enthalpy tables (see Appendix 1) when the prduct gas cmpsitin is knwn frm equilibrium calculatins. Example 1.3 The heat input fr a tubular refrmer in a hydrgen plant (100,000 Nm 3 /h) shuld be estimated. The utlet equilibrium prperties f the feed were calculated in Example 1., s this example is a cntinuatin f this with the purpse f calculating the heat balance ver the tubular refrmer. The ttal inlet and utlet atmic balances fr C and O are set up. The C balance can be used t calculate the ttal utlet flw by use f the wet cmpsitin fund in Example 1.. Similarly, the O balance can be used t calculate the water inlet flw as fllws: C : CH 4,in CH 4,ut CO ut CO,ut O : H Oin COut CO,ut H Out gives Fut 459 ml s H Oin 150 ml s CH 4,ut 146 ml s H,ut COut 1417 ml s The steam-t-carbn rati in the feed is thus: H O/CH 4 =.5 and the ttal cnversin f CH 4 is 71%. If all CO in the prduct can be cnverted t H in the subsequent shift reactin system, the ttal amunt f prduct is 114,338 Nm 3 /h f which 100,000 Nm 3 /h r 88% in this case is recvered as final prduct. A ttal heat balance ver the tubular refrmer can nw be established by using the calculated cmpnent flws and the crrespnding enthalpies f frmatin f the feed cmpnents at 500ºC and the prduct cmpnents at 875ºC frm Appendix 1 as:

31 Rutes t Syngas 33 Inlet : Outlet : Q added 4.88 H O H O in ( ) CO ( 51.75) CH ut,ut ( 5.47) CH 5.0 H 4,in ut kJ s 4,ut ( 83.88) CO kJ s ( ) kJ s 108MW If an average heat f reactin equal t 5 kj/ml (valid at 700ºC) is used, it is fund that in this case 75% f the ttal heat added is used in the reactin and the rest is used fr heating f feed and prduct. The split between the tw parts changes, f curse, with H O/CH 4 rati and ther perating cnditins. ut The catalyst is nrmally nickel n a stable supprt (refer t Chapter 4). The catalyst prperties are dictated by the severe perating cnditins, including temperatures f C and steam partial pressures f up t 30 bar. The intrinsic activity f the catalyst depends n the nickel surface area. The catalyst is placed in a number f high-ally refrming tubes placed in a furnace as shwn in Figure Tubular refrmers are designed with a variety f tube and burner arrangements (as shwn in Sectin 3..). Such refrmers are built tday fr capacities up t mre than 300,000 Nm 3 f H (r syngas) /h. Waste heat sectin Refrmer furnace Only every secnd tube (f 66) is shwn Figure 1.13 Refrmer furnace and waste heat sectin [45]. Reprduced with the permissin f Wiley.

32 34 Cncepts in Syngas Manufacture Heat transfer takes place primarily (>95%) by radiatin frm the furnace gas and in a sidewall-fired furnace als frm the furnace walls. The remaining transfer is by cnvectin. Abut 50% f the fuel cmbustin heat is transferred thrugh the tubes fr the refrming reactins and fr heating up the gas t the exit temperature. The remaining cmbustin heat is recvered in the waste heat sectin (Figure 1.13). It is pssible t increase the amunt f heat transferred t the prcess gas in the refrmer frm abut 50% t abut 80% f the supplied heat when using a cnvective heat exchange refrmer in which the flue gas as well as the ht prduct gas are cled by heat exchange with the prcess gas flwing thrugh the catalyst bed. This results in a mre cmpact piece f equipment [171]. Hwever, in all types f heat-exchange refrmers, the heat exchange is by cnvectin, and this generally leads t lwer average heat fluxes t prtect the cnstructin materials than in refrmers with radiant heat transfer. Therefre, in principle the fired tubular refrmers may appear the mst ecnmic slutin fr large-scale peratin, but the cnvective refrmer may be applied in cmbinatin with the tubular refrmer fr mre efficient heat recvery by chemical recuperatin [46] [47]. This is further discussed in Sectin. and Chapter Carbn frmatin. Higher hydrcarbns Steam refrming invlves the risk f carbn frmatin (Table 1.4). The frmatin f carbn may lead t breakdwn f the catalyst and the buildup f carbn depsits and disintegrated catalyst pellets may cause partial r ttal blckage f the refrming tubes resulting in develpment f ht spts r ht tubes [389]. The parameters determining the risk f carbn frmatin are discussed in Chapter 5. Higher hydrcarbns shw a higher tendency fr carbn frmatin n nickel than des methane and, therefre, special catalysts either cntaining alkali r rare earths r based n an active magnesia supprt are required (refer t Sectin 5.3.) [389] [45]. Naphtha can be prcessed directly in the tubular refrmer, as practiced in many industrial units, but the cntrl f the preheat

33 Rutes t Syngas 35 temperature and heat flux prfile may be critical [384]. This is a severe cnstraint as the heat required in the tubular refrmer and hence the refrmer csts may be reduced by increasing preheat temperature. Hwever, the preheater may then wrk as a steam cracker prducing lefins frm higher hydrcarbns in the feed [53] (refer t Sectin 4.3.3). The lefins easily frm carbn in the refrmer. Apart frm the pressure, the cnditins in the tubular steam refrmer and in the preheater are nt far frm that f a steam cracker in an ethylene plant. These cnstraints are remved when using an adiabatic prerefrmer [394] [415] as illustrated in Figure The prerefrming catalyst is typically a highly active nickel catalyst. This catalyst als wrks as an effective sulphur guard fr the tubular refrmer and dwnstream catalysts, by remving any traces f sulphur still left after the desulphurisatin sectin. Figure 1.14 Flw diagram f prcess with tubular refrmer with prerefrmer [394]. Reprduced with the permissin f Elsevier. With prper desulphurisatin, it is pssible t cnvert naphtha and heavy distillate feedstcks such as kersene and diesel in a prerefrmer int syngas with n trace f higher hydrcarbns [389] [405].

34 36 Cncepts in Syngas Manufacture All higher hydrcarbns are cnverted in the prerefrmer in the temperature range f C, and the refrming and shift reactins are brught int equilibrium. After a prerefrmer, it is pssible t preheat t temperatures arund 650 C, thus reducing the size f the tubular refrmer. A revamp f an ammnia plant [457] by installing a prerefrmer and larger preheater invlved the increase in the refrmer inlet temperature frm 50 C t 650 C. This resulted in reduced fuel cnsumptin. With lw catalyst activity, the thermal cracking rute (pyrlysis) may als take ver in the refrmer tube [389]. This is the situatin in case f severe sulphur pisning r in attempts t use nn-metal catalysts with lw activity. The risk f carbn frmatin depends n the type f hydrcarbn with the cntents f armatics being critical. Ethylene frmed by pyrlysis results in rapid carbn frmatin n nickel (refer t Sectin 5.). Ethylene may als be frmed by xidative cupling if air r xygen is added t the feed r by dehydratin f ethanl Nn-tubular refrming In a tubular refrmer, the tube diameter is selected frm the mechanical cnsideratins leaving the space velcity (catalyst vlume) as a dependent parameter. This s-called tubular cnstraint can be illustrated by the simple example: Nte Fr given tube length, L, flw, F, and transferred heat (refrmer duty), Q, the number f tubes, n, is determined by the tube diameter,: d t, the average heat flux,: q av and the space velcity,: SV (Nm 3 /h/m 3 cat). Fr cnstant inlet and utlet cnditins, this means: Q ndtlqav Q F SV n d t L 4 (1.1) frm which : q SV d av t The tubular cnstraint (the last equatin) can be made less restrictive by cnvective heat exchange refrmers. This may invlve the use f

35 Rutes t Syngas 37 catalysed heat transfer surfaces [188] [397] [488] in the frm f plate type refrmers and multi-channel refrmers. This is discussed further in Sectin Anther apprach is t decuple the heat transfer and the reactin. This includes reheat schemes [403] [455] [548] in which the prcess gas is heated in a heater fllwed by refrming reactin in an adiabatic reactr as illustrated in Figure Hwever, many steps are required t reheat the gas because f the strng endthermicity f the reactin. Figure 1.15 Reheat scheme fr steam refrming [403]. Reprduced with the permissin f Japan Petr. Inst. A variatin f the reheat prcess scheme is the use f a circulating catalyst bed using ne bed fr reactin and the ther fr heating up the catalyst [549]. This is als applied in ther fluidised petrchemical prcesses [439]. Hwever, fr steam refrming the recirculatin rate wuld be very high. Mrever, catalyst dust in dwnstream heat exchangers wuld result in methane frmatin by the reverse refrming reactin (methanatin). Other attempts have aimed at utilising the high heat transfer in fluidised beds and supplying the heat by an external heater [10]. Other suggestins [6] have dealt with supplying the heat by additin f a CO acceptr (CaO, etc.) t the fluidised bed. The heat

36 38 Cncepts in Syngas Manufacture frm the frmatin f carbnate is almst sufficient fr the refrming reactin [6]. An alternative t the refrming prcess may be the use f a cyclic prcess [438] as illustrated in Figure Hydrgen is generated by reacting steam with a metal (Cu, Fe, etc.). The resulting metal xide is reduced by reactin with methane-frming steam and CO at a pressure well suited fr sequestratin. The scheme invlves a number f cnstraints relating t heats f reactin. The additin f air is necessary t ensure that the verall reactin becmes thermneutral. CH 1.3H O 0.34O 3.3H CO H 0 (1.13) 4 98 Figure 1.16 Cyclic prcess fr CO -free hydrgen [414]. Reprduced with the permissin f Balzer. 1.3 Other manufacture rutes Partial xidatin An alternative apprach t steam refrming is t add xygen t the feed and hence gain the necessary heat by internal cmbustin. It means that the steam frmed by the cmbustin is cndensed in the prcess stream instead f leaving as water vapur in the flue gas frm the fired refrmer. Hence, the higher heating value f the fuel is recvered in the partial

37 Rutes t Syngas 39 xidatin schemes in cntrast t the lwer heating value in the steam refrming prcess. Partial xidatin can be carried ut in three different ways as illustrated in Figure 1.17: Nn-catalytic partial xidatin (POX); Autthermal refrming (ATR); Catalytic partial xidatin (CPO). Figure 1.17 Syngas by partial xidatin. Temperature prfiles fr varius rutes t syngas by partial xidatin [398]. Reprduced with the permissin f Elsevier. The nn-catalytic partial xidatin [486] (POX, Texac, Shell) needs high temperature t ensure cmplete cnversin f methane and t reduce st frmatin. Sme st is nrmally frmed and is remved in a separate st scrubber system dwnstream f the partial xidatin reactr. The thermal prcesses typically result in a prduct gas with H /CO= Gasificatin f heavy il fractins, petcke, cal and bimass may play an increasing rle as these fractins are becming mre available and natural gas (NG) less available. The autthermal refrming (ATR) prcess is a hybrid f partial xidatin and steam refrming using a burner and a fixed catalyst bed fr

38 40 Cncepts in Syngas Manufacture equilibratin f the gas. This allws a decrease in the maximum temperature and hence the xygen cnsumptin can be lwered. On the ther hand, the high temperature in POX units may result in lwer cntents f methane and carbn dixide in the prduct gas than in ATR. St frmatin in ATR can be eliminated by additin f a certain amunt f steam t the feedstck and by special burner design. Steam can hardly be added t the nn-catalytic prcesses withut the risk f increased st frmatin because f the resulting lwer temperature. This means less flexibility fr the cmpsitin f the syngas frm POX units. In catalytic partial xidatin (CPO) the chemical cnversins take place in a catalytic reactr withut a burner. In all cases f partial xidatin, sme r all f the reactins listed in Table 1.7 are invlved. The partial xidatin reactins are accmpanied by the steam refrming and shift reactins (Table 1.). The xidatin reactins are irreversible under all cnditins f practical interest. Table 1.7 Reactins ccurring in partial xidatin f methane. Reactin H 98 kj ml R10 CH4 0.5O CO H 36 R11 CH4 O COHO H 78 R1 CH4 1.5O CO HO 519 R13 CH4 O CO HO 80 The verall reactin in a partial xidatin reactr is strngly exthermic and n heat must be supplied t the reactr. The partial xidatin reactins may be accmpanied by cracking f the hydrcarbns r by xidative dehydrgenatin int nn-saturated cmpunds including lefins, ply-armatics and st. The cntrl f the heat balance and the frmatin f by-prducts are main cnsideratins in the design f partial xidatin reactrs.

39 Rutes t Syngas 41 Tw-step refrming features are a cmbinatin f tubular refrming (primary refrmer) and xygen-fired secndary (autthermal) refrming. In this cncept the tubular refrmer is perating at less severe peratin, i.e. lwer utlet temperatures (refer t Sectin.6.) Autthermal refrming The ATR technlgy was pineered by SBA and BASF in the 1930s [413], by Tpsøe and SBA in the 1950s [331], and later Tpsøe alne [107] [111] develped the technlgy at first fr ammnia plants and later fr large-scale gas-t-liquid plants. Tday, ATR is a cst-effective technlgy fr the synthesis gas sectin fr a variety f applicatins [187]. Figure 1.18 ATR reactr.

40 4 Cncepts in Syngas Manufacture The ATR reactr cnsists f a burner, a cmbustin chamber, and a fixed catalyst bed placed in a cmpact refractry lined vessel [107] as illustrated in Figure Irrespective f whether the burner is thermal r catalytic r whether a fixed r a fluidised catalyst bed is used, the prduct gas will be determined by the thermdynamic equilibrium at the exit temperature, which in turn is determined by the adiabatic heat balance. The feedstcks are hydrcarbns, steam, and either xygen r air (r a mixture theref). Optinally, carbn dixide may be added t the hydrcarbn stream (refer t Sectin.4.). The mixture f hydrcarbn and steam is preheated and mixed with xygen in the burner. An adiabatic prerefrmer may be advantageus t eliminate thermal cracking f higher hydrcarbns in the preheater. A turbulent diffusin flame ensuring intensive mixing is essential t avid st frmatin. The burner is designed t avid excessive metal temperatures in rder t ensure lng lifetime [109]. Typically, the mlar rati f xygen (as O ) t carbn in the hydrcarbn feed stream is with xygen as the xidant [111] and the temperature f the flame cre may be higher than 000C. Hence, the design f the cmbustin zne is made t minimise transfer f heat frm the flame t the burner [107]. Thermal cmbustin reactins are very fast. The sub-stichimetric cmbustin f methane is a cmplex prcess with many radical reactins [466]. The reactin pattern depends n the residence time/temperature distributin. Hence, it is imprtant t cuple the kinetic mdels with CFD simulatins by pst prcessing [466] r by direct cupling in mre advanced calculatins. The sub-stichimetric cmbustin invlves the risk f st frmatin as a result f pyrlysis reactins with acetylene and plycyclic armatic hydrcarbns as st precursrs [107] [465]. The st frmatin will start belw a certain steam-t-carbn rati depending n pressure and ther perating parameters [111]. Hwever, the data in Table 1.8 shws results frm a st-free pilt test (100 Nm 3 NG/h) at a lw steam-t-carbn rati f 0.1 [111]. The methane steam refrming and shift reactins (Reactins R1 and R4 in Table 1.) als ccur thermally in the cmbustin zne, but far

41 Rutes t Syngas 43 frm equilibrium fr the refrming reactin, whereas the shift reactin remains clse t equilibrium. Table 1.8 ATR pilt test at lw H O/C. H O/C=0.1, O /C=0.59, P=4.5 bar, T exit =1057 C. Prduct gas analysis vl% [111]. H N CO CO CH 4 H O % H /CO=1.96, selectivity t H +CO (dry gas): 95%. St precursrs and residual methane are cnverted by steam refrming and shift reactins in the catalyst bed. These reactins are in equilibrium in the gas leaving the catalyst bed and the ATR reactr. The catalyst size and shape is ptimised t have sufficient activity and lw pressure drp t achieve a cmpact reactr design. The catalyst must be able t withstand the high temperature withut excessive sintering r weakening and it shuld nt cntain cmpnents vlatile at the extreme cnditins. A catalyst with nickel n magnesium alumina spinel has prven t fulfil these requirements [107] [111]. Specific studies [3] have been made n the use f ATR fr synthesis gas prductin fr very large methanl and FT plants, cnsidering the limitatin by ther parts f the plant, e.g. bilers, cmpressrs, air separatin units, etc Catalytic partial xidatin In catalytic partial xidatin (CPO), the reactants are premixed and the reactins prceed n the catalytic reactr withut a burner, in cntrast t ATR [184]. The principle f a fixed-bed catalytic partial xidatin (CPO) is illustrated in Figure CPO with n flame was practiced in the 1950s [355] at lw pressure and later by Lurgi at pressure [31]. The reactin was started by an

42 44 Cncepts in Syngas Manufacture ignitin catalyst. The additin f xygen t a fluidised bed steam refrmer has been studied by a number f grups [53] [49] including pilt scale peratin [180] and lately with hydrgen membranes [100] [341]. Figure 1.19 Principle f catalytic partial xidatin (CPO). Extensive studies f CPO reactins were carrried ut by Lanny Schmidt et al. [9] [443] using a millisecnd fixed-bed reactr. It was pssible t prduce syngas ver a rhdium mnlith at residence times f millisecnds [9]. Platinum was less active than rhdium. It was shwn [4] that the reactins take place in an xidatin zne as cmbined surface/gas-phase reactins fllwed by a steam refrming zne with equilibratin f the steam refrming and shift reactins. It was als pssible t cnvert liquid hydrcarbns [164], ethanl [434] and bimass [444] in the millisecnd reactr. The fixed-bed CPO technlgy has been studied at pilt scale [41] and at pressure. When perating at 0 bar [] [41] and O /NG=0.56, it was pssible t achieve stable cnversins clse t thermdynamic equilibrium.

43 Rutes t Syngas 45 The direct CPO reactin R10 in Table 1.7 appears t be the ideal slutin fr methanl and Fischer Trpsch syntheses, as it prvides a H /CO mlar rati f and has a lw heat f reactin: CH O CO H (1.14) It is ften cnsidered a dream reactin with H /CO ratis lwer than tw at lw cnversins, but industrial utilisatin wuld imply expensive recycle f nn-cnverted methane as als discussed fr direct cnversin f methane (refer t Sectin 1.1.). Other studies which claim high yields at lw temperatures may be misleading [105]. Figure 1.0 CPO catalyst and perating cnditins [410]. In a flat bed reactr the catalyst temperature may easily be significantly higher than the gas temperature, whereas the catalyst and gas temperatures may fllw each ther in a reactr perating at high Reynld numbers. Reprduced with the permissin f Springer. Nte High yields f carbn mnxide and hydrgen were claimed in CPO studies at reactr temperatures as lw as 300ºC [105] with prduct gas cmpsitins crrespnding t equilibrium f the refrming reactin at ºC. Hwever, it culd be estimated [153] [410] that the catalyst surface might have had an ver-temperature f ºC because f strng film diffusin due t lw Reynlds number N Re. Therefre, the

44 46 Cncepts in Syngas Manufacture prduct gas mst likely reflects the gas equilibrium at the catalyst surface. At lw N Re in a flat reactr perating with lw cnversin, the catalyst temperature may easily be higher than the gas temperature and clse t that resulting frm the adiabatic temperature increase at full cnversin as illustrated in Figure 1.0. In an integral reactr perating at high N Re, the temperatures f the gas and the catalyst will increase simultaneusly thrugh the bed. Measurements f cnversins at CPO cnditins reprted in the literature are seldm shwing cnversins relative t the equilibrium cnversins. Yields are ften expressed as selectivities t hydrgen and carbn mnxide althugh the prduct gas in mst situatins is clse t equilibrium fr the refrming and shift reactins. Therefre selectivity data shuld be supplemented by calculatin f apprach t equilibrium, ΔT ref (Equatin 1.11). Reprted selectivities may be misleading. Even sphisticated catalyst cmpsitins lead t equilibrated prduct gas as illustrated in Example 1.4 belw. Example 1.4 As an example, ne study [7] claimed high selectivity t CO+H using a sphisticated catalyst at 775ºC. Equilibrium calculatins are carried ut fr a stichimetric mixture f CH 4 and O reacting at 775ºC and ambient pressure accrding t: P P H CO 11 CH O CO H K eq,x P P O CH 4 Frm an equilibrium pint f view all xygen is reacted, but due t the presence f a catalyst, equilibrium is als established in the steam refrming and shift reactins: CH 4 H O CO 3H CO H O CO H K K eq,ref eq,shf p p p p H CH CO 3 H 4 p p p p CO CO H O H O Values f the equilibrium cnstants are calculated using the data given in Appendix. It is seen that full cnversin can be assumed in the xygen reactin, s after slving the tw remaining equatins fr

45 Rutes t Syngas 47 the unknwn cnversins using the principle in Example 1.1, the fllwing equilibrium cmpsitin is fund: CH4 3.53, H 6.08, HO.4, CO and CO 1.9. This crrespnds t a ttal cnversin f CH4 equal t 90% and selectivities t CO and H equal t: CH 4,CO CO % CO CO CH 4,H H % H H O This is in fine agreement with measurements such as thse published in [7] fr CPO ver transitin metal catalysts. It shuld be nted that adding an inert makes the equilibrium even mre favurable due t the decrease in partial pressures. As shwn in Table 1.8, the ATR prcess at lw steam carbn rati is very clse t fulfilling H/CO= at selectivities abve 90% fr CO H. Figure 1.1 The dark reactr shws a CPO pilt (10 Nm3/h CO+H and 5 bar) in frnt f a tubular refrmer f twice the capacity. The size f the CPO catalyst bed is indicated. Therefre, the prduct gas frm CPO at high cnversins will be clse t the thermdynamic equilibrium f the steam refrming and water-gas-shift reactins [4]. Fr adiabatic peratin, the exit