Jess 1999

Applied Catalysis A: General 186 (1999) 321342

FischerTropsch-synthesis with nitrogen-rich syngasFundamentals and reactor design aspects

A. Jess a;, R. Popp b, K. Hedden ba Institut fr Technische Chemie und Makromolekulare Chemie, RWTH Aachen, Worringer Weg 1, D-52074 Aachen, Germany

b Engler-Bunte-Institut, Bereich Gas, Erdl und Kohle, Universitt Karlsruhe (TH), Richard-Willsttter-Allee 5, D-76131 Karlsruhe, Germany

Abstract

An option in bringing remote natural gas reserves to the market is its conversion by FischerTropsch (FT)-synthesis intodiesel oil and wax. The use of nitrogen-rich syngas (50 vol.%) could be an alternative to classical processes with nitrogen-freesyngas because the investment costs are probably lower: syngas is produced by partial oxidation with air, which eliminatesthe need for an air separation plant, and a process with nitrogen-rich syngas does not utilize a recycle loop and a recyclecompressor.

For the development of such a process, the kinetics of FT-synthesis was studied on an Fe-catalyst, indicating that nitrogenonly dilutes syngas, and therefore, has no influence on the kinetics if the partial pressures of carbon monoxide and hydrogenare kept constant. Subsequently, the FT-synthesis with nitrogen-rich syngas was investigated in wall-cooled single tubereactors.

Based on the experimental data, a mathematical model for industrial multitubular FT-reactors was developed. Modelcalculations indicate that nitrogen plays an important role in the operation of multitubular reactors by helping to remove theheat generated by the FT-reaction. This leads to an optimum diameter of the tubes of 70 mm for nitrogen-rich syngas withrespect to a stable and safe operation of the reactor, whereas for nitrogen-free syngas, the diameter is limited to about 45 mm.The production rate of diesel oil and wax per tube is, in case of nitrogen-rich syngas, about three times higher, which willdecrease the number of tubes and the investment costs of industrial multitubular reactors. Detailed economic studies are stillnecessary to validate or disprove whether and under which circumstances the proposed process with nitrogen-rich syngas isan attractive alternative to classical processes with nitrogen-free syngas, especially in areas with remote natural gas resources.1999 Elsevier Science B.V. All rights reserved.

Keywords: FischerTropsch (FT)-synthesis; Iron-catalyst; Nitrogen-rich syngas; Fixed bed reactor; Multitubular reactor; Parametricsensitivity

1. Introduction

About 50% of the proven natural gas reserves aretoday distant from any sizeable market [1], e.g. large

Corresponding author. Tel.: +49-241-80-6470; fax:+49-241-8888177E-mail address: [email protected] (A. Jess)

reserves in the Middle East (Table 1). The economicutilization of this natural gas is limited by its costlytransportation. An option in bringing remote naturalgas to the market is its conversion by FischerTropsch(FT)-synthesis into higher hydrocarbons like dieseloil and wax. These products are of high value and caneasily be shipped, obviating the need for dedicatedcryogenetic transportation, equipment and tankage.

0926-860X/99/$ see front matter 1999 Elsevier Science B.V. All rights reserved.PII: S0926 - 860 X (99 )00152 -0

322 A. Jess et al. / Applied Catalysis A: General 186 (1999) 321342

Table 1Global natural gas reserves [2]Country Natural gas reserves in 1012 m3 (STP)

1985 1996

CIS (former USSR) 42:5 55:9Iran 13:3 21:0Qatar 4:2 7:1United Arab Emirates 0:9 5:8Saudi-Arabia 3:4 5:3USA 5:6 4:7Venezuela 1:7 4:0Algeria 3:0 3:7

World 98:8 138:1

The FT-synthesis can be summarized by the fol-lowing reactions:

CO C 2H2 ! CH2 C H2OIRH

0298 D 152 kJ=mol (1)

CO C 3H2 CH4 C H2OIRH

0298 D 206 kJ=mol (2)

CO C H2 CO2 C H2I RH 0298 D 41 kJ=mol(3)

The first two equations describe the formation ofhigher hydrocarbons and of methane, respectively;the CH2 group in Eq. (1) indicates a link of a hy-drocarbon molecule. Carbon monoxide can also reactwith water vapor, which is formed (Eqs. (1) and (2))or already present in syngas, to carbon dioxide andhydrogen (Eq. (3)).

Since the mid of the 1950s up to the beginning ofthe 90s, the FT-synthesis was operated industriallyonly in South Africa and exclusively with syngas gen-erated by coal-gasification [3]. In recent years, a re-naissance of the FT-synthesis based on syngas fromnatural gas can be stated, which takes into account thegrowing importance of natural gas, in particular of re-sources, which are remote from the market: Since 1992, liquid fuels are being produced from

natural gas in the Mossel Bay gas conversion projectin South Africa [4,5]. In a first step, syngas is gener-ated by steam reforming as well as by partial oxida-tion of natural gas with oxygen. The combined syn-gas streams are converted into liquid fuels (0.9 Mt

per annum) by FT-synthesis on an iron-based cat-alyst in circulating fluidized-bed reactors.

A second industrial FT-plant for the production ofliquid hydrocarbons (about 0.5 Mt per annum) fromnatural gas is in operation in Malaysia since 1993[4]. The so-called Shell Middle Distillate Synthe-sis (SMDS)-process, developed by Shell, uses mul-titubular FT-fixed bed reactors and a co-catalyst[6]. The main product is wax, which is further con-verted by hydrocracking into diesel oil.

Exxon announced in 1994 the development of aso-called second generation process, the AdvancedGas Conversion (AGC-21) process [1]. Syngas isgenerated by the catalytic conversion of natural gaswith oxygen and steam in a fluidized bed reactor,where partial oxidation and steam reforming takeplace simultaneously. Syngas is subsequently con-verted on a co-catalyst in a FT-slurry reactor. Themain product is wax, which is further cracked toliquid fuels. The process was tested in a pilot plant.

In 1996, South African Coal, Oil and Gas Corp.Ltd. (Sasol) and Haldor Topsoe from Denmark an-nounced a cooperation agreement for combined pro-motion of Sasols slurry phase FT-technology andHaldor Topsoes technology for the conversion ofnatural gas to syngas [7].

In the 90s, Rentech Inc. (USA) was operating asmall FT-unit, which uses an iron catalyst in aslurry reactor to produce gasoline, diesel oil andwax (about 15,000 tons per annum) [4]. In 1996,this plant was sold, dismantled and shipped to itsIndian licensee, Donyi Polo Petrochemicals. It isnow installed in northeastern India and should bein operation in the near future. The plant will useabout 100,000 m3 (STP) per day of presently flaredgas.

In 1997, Qatar General Petroleum Corp. startednegotiations with Sasol for the construction of anFT-plant in Qatar with an annular capacity of about1 Mt middle distillates. In addition, talks are goingon with Exxon about a plant with a capacity of 5 Mtper annum [8].

Also in 1997, Sasol and the Norwegian companyStatoil announced a cooperation for the constructionof small floating FT-plants in order to utilize oilassociated gas offshore in areas of the North Sea,where it is uneconomical to build a pipeline for thegas [8].

A. Jess et al. / Applied Catalysis A: General 186 (1999) 321342 323

Although very little data are available from the lit-erature, the production costs of these capital-intensiveprocesses are probably above the break-even point.

In contrast to the common philosophy of construct-ing chemical plants of high efficiency, i.e. of high in-vestment costs in order to keep energy and feedstockcosts down, we are convinced that, for countries inremote areas with low prices of energy of natu-ral gas the only economical solution for the con-version of natural gas to higher hydrocarbons is alow-cost technology, even if the conversion efficiencyis lower [9,10]. The concept of the following low-costFT-process based on nitrogen-rich syngas (Fig. 1)was investigated on a semi-technical scale from 1987to 1997 at the Engler-Bunte-Institute of the Universityof Karlsruhe.

After desulphurization, the natural gas is con-verted into a nitrogen-rich (about 50 vol.% nitrogen)and soot-free syngas by catalytic partial oxidationwith air at a pressure of about 25 bar. Using airinstead of oxygen eliminates the need for an airseparation plant. The autothermal reactor is (on in-dustrial scale) a refractory-lined vessel utilizing anickel-based catalyst. This reactor is comparedto a steam-reformer relatively inexpensive. In ad-dition, no soot-separation is needed [11,12] as fornon-catalytic partial oxidation with pure oxygen,which is used both in the Mossel Bay project and theSMDS-process.

Without further compression, the nitrogen-richsyngas is converted by FT-synthesis in multitubularfixed bed reactors to higher hydrocarbons. A commer-cial precipitated iron catalyst (Hoechst-Ruhrchemie,Germany), similar to the one in the common industrialfixed bed FT-process, the so-called ARGE-process(Sasolburg, South Africa, coal-based syngas), wasused in the experiments. In contrast to this pro-cess, which uses nitrogen-free syngas, the proposedlow-cost process with nitrogen-rich syngas (about50%) does not utilize a recycle loop to avoid anybuild-up of nitrogen in the system. This configurationis probably less expensive because a recycle compres-sor is not needed. In addition, the nitrogen in syngasplays a significant role by helping to remove the largeamounts of heat generated by the FT-reaction, whichwill be discussed in detail in this paper. The productsare separated by cooling down into organic conden-sates (wax and diesel oil), water and an off-gas, which

may be used to generate power, and in countries likein the Middle East, to desalinate sea water.

The task of the project was to develop and to test thislow-cost process on a semi-technical scale in order toobtain the basic data for further economic evaluations.

The concept of this process, which was invented byHedden in 1987 [9], and experimental data of syngasgeneration and of the FT-synthesis on lab-scale aswell as in semi-technical reactors have already beendiscussed in detail elsewhere [1018]. Therefore,only some major results of the final experiments withthe semi-technical plant for the production of liquidhydrocarbons from natural gas are subsequentlypresented.

2. FT-synthesis with nitrogen-rich syngas:experiments on a semi-technical scale

2.1. Experimental set-up

The experiments were carried out with thesemi-technical unit shown in Fig. 2. Briefly, the ap-paratus consists of the syngas generation unit (upto 10 m3 syngas (STP)/h; details in [11,12]), theFT-unit with three single-tube fixed bed reactors inseries (details in [16]), and the product separation.In some experiments, e.g. where the steam contentof syngas was varied, syngas was made by mixingits components. The diameter of the semi-technicalFT-reactors corresponds to a single tube of a typicalindustrial multitubular reactor (Fig. 3); the experi-mental results are, therefore, directly transferable to atechnical scale.

The FT-synthesis is highly exothermic (see Eqs.(1) and (2)) with an enthalpy of about 20 % of theone released by the total oxidation of syngas. Theheat removal and control of the reaction temperatureare, therefore, extremely critical steps since the dam-age of the catalyst at temperatures above 260C [14]decreases the conversion rate. In addition, the prod-uct distribution shifts with increasing temperature to-wards the production of more unwanted methane 1 . In

1 The hydrocarbon distribution of the FT-synthesis is governedby the so-called probability of chain growth, which generally de-creases with increasing temperature. So more short-chain hydro-carbons, and therefore, less diesel oil and wax are formed at highertemperatures. Details of the Fe-catalyst used here are given in[1315,18].


Fig. 1. Simplified block diagram of the proposed low-cost process for the conversion of natural gas to higher hydrocarbons based onnitrogen-rich syngas.

Fig. 2. Simplified flow scheme of the semi-technical unit for the conversion of natural gas to higher hydrocarbons (R: FT-reactor, HS: hotseparator, S: separator, H: heater, C: condenser; for details of desulphurization and syngas generation see [11,12]; for the FT-synthesisunit, see [16]).

industrial FT-fixed bed reactors, this is avoided byusing multitubular reactors, which are cooled by pres-surized boiling water. In the semi-technical reactors,this was obtained by cooling with oil (Fig. 3).

Syngas passes through the single tube reactors and

is converted at a total pressure of about 24 bar andtemperatures of 220C up to 260C. The product gasis cooled in several steps, whereby the organic con-densates (wax, diesel oil, (heavy) gasoline) and waterare separated.


Fig. 3. Comparison of a common technical multitubular FT-reactor with the semi-technical single tube reactor used in the experiments.

2.2. Experimental results

The semi-technical FT-plant was successfullyrun in several experiments for a period of alto-gether about 1400 h (8 weeks), thereby using syn-gas obtained by mixing its components. Finally, thecomplete semi-technical plant for the productionof higher hydrocarbons from natural gas (desul-phurization of natural gas syngas generation FT-synthesis product separation, see Fig. 2) wasrun for 1 week without any interruption [16,17]. TheCO-conversion in the FT-unit was 55%. A typicalproduct-distribution is shown in Fig. 4, indicating that,under the given reaction conditions, diesel oil and waxare the main organic products of FT-synthesis. Theselectivity to the (unwanted) product carbon dioxideis about 25 C%, which is typical for iron-catalysts.

The diesel oil with a cetane number >70 has a verygood quality (standard value according to DIN 51601:45). The diesel oil and wax contain no sulphur or otherimpurities and the wax is an excellent feed for hy-drocracking to produce middle distillates like dieseloil and jet fuel. The diesel oil may also be used forblending to fit present and after all future state regu-lations. Very little gasoline was obtained, only about15 C%, of the total organic condensates. The quality

Fig. 4. Typical product-distribution of FT-synthesis obtained withthe semi-technical FT-unit as shown in Fig. 2 (Note that CO2and CH4 already present in syngas are not counted as products;temperature (depending on axial and radial position): 225260C;for details see [16,17]).

of FT-gasoline with a research octane number of lessthan 0 is very poor (standard value according to DIN51600: >91). Therefore, the (heavy) gasoline fractionwas added to the diesel oil fraction, which improves itsquality by increasing the boiling-point curve to stan-dard values [16]. The off-gas, which contains not onlymore than 60 vol.% nitrogen, but also unconverted car-bon monoxide and hydrogen, methane and higher hy-drocarbons, has a low net calorific value Hu of about4 MJ/m3(STP), and can be used to generate power andto desalinate sea water.


Fig. 5. Steady state-equations of the model for the FT-reactor.

3. Reactor modeling

3.1. General

The processes in the single tube of a multitubularFT-reactor can be described by a model based on acomplex set of equations that encounter the heat andmass transfer as well as the kinetics of the main re-actions. The purpose of the model was to determinethe temperature and concentration profiles of the reac-tor. The validity of the model was proven by compar-ison of the computed and experimental results, which

were obtained with the semi-technical unit. Based onthe reactor model (and the corresponding computerprogram, which are, in the following, synonymouslydenoted as the model), the influence of differentparameters (steam and nitrogen content of syngas,tube diameter, volume rate, cooling temperature) onthe behavior of industrial multitubular FT-reactorswas studied. Subsequently, the major results of re-actor modeling will be discussed, especially with re-spect to the performance of industrial multitubularFT-reactors if nitrogen-free or nitrogen-rich syngas isused.


The so-called parameteric sensitivity was also con-sidered to determine the allowable diameter of thetubes, i.e. a further increase in the diameter would leadto a situation where the reactor gets very sensitive tosmall unavoidable fluctuations of the reaction condi-tions (e.g. the cooling temperature), and so a temper-ature runaway is likely. In the following (Section 4.3),the results of the parametric sensitivity towards thecooling temperature are presented.

3.2. Detailed description of the reactor model

According to the common classification of fixedbed reactor models [19], the model used here is apseudo-homogeneous two-dimensional model, therebyneglecting the axial dispersion of mass and heat aswell as the radial concentration gradients. The radialtemperature profile is considered, and the correspond-ing heat transfer in radial direction within the catalystbed is expressed in terms of an effective heat conduc-tion with a constant effective radial heat conductivityrad;eff . To calculate the heat transfer from the cata-lyst bed to the reactor wall, an inner wall heat transfercoefficient W;i has to be introduced to account forthe particular conditions of the heat transfer near thewall. In addition, the heat conductivity of the wall Wand the outer wall heat transfer coefficient W;o (wallto cooling medium) have to be considered. The coef-ficients rad;eff , W;i, and aW;o were calculated withstandard correlations taken from the literature (rad;eff[20], W;i [21,22], and W;o [23]). The basic equa-tions of the model are listed in Figs. 5 and 6. Detailsof the model and the computer program are given in[16].

The total rate of CO-consumption 6rCO;i(in Fig. 6)in the complex reaction system of FT-synthesis canbe described by three parallel main reaction paths andtheir respective reaction rates: formation of C2C-hydrocarbons and H2O (Eq. (1)

and Eq. (7)), formation of CH4 and H2O (Eq. (2) and Eq. (8)),

and formation of CO2 and H2 (Eq. (3) and Eq. (9)).

The kinetic equations and parameters (reaction or-ders, activation energies etc., see Fig. 6) were deter-mined by classical isothermal methods in lab-scale re-actors [1315], and in the case of methanation andshift reaction by the best fit of the model with the ex-

perimental results of the semi-technical reactor [16];the latter two kinetic equations should, therefore, beproved by further measurements.

Thereby must be noted that experiments with syn-gases containing different amounts of nitrogen clearlyindicate that nitrogen only dilutes syngas and has noinfluence on the kinetics of the synthesis, if the partialpressures of carbon monoxide and hydrogen are keptconstant [13,14]. It is also important to note that thekinetic parameters listed in Fig. 6 (e.g. kFT) considerpore diffusion limitations and only refer to the parti-cle diameter of 2.5 mm as used in the semi-technical(as well as in industrial) FT-reactors: after a shortstart-up period with a fresh catalyst, the pores of thecatalyst are completely filled with liquid higher hy-drocarbons (wax) [15,18], which leads to a relativelyslow diffusion of carbon monoxide and hydrogen in-side the pores, and in turn, to an effective depth ofpenetration of the catalyst particles by carbon monox-ide of only about 0.3 mm (compared to the particlediameter of about 2.5 mm) at temperatures of about250C. The effectiveness factor, defined as the ratio ofthe actual rate of reaction in the particle as a whole,to the rate, which would be expected, if there were noconcentration gradients inside the particle, is, there-fore, only about 0.2 (at 250C). As expected in therange of pore diffusion, the apparent activation ener-gies given in Fig. 6 are only half of the true activationenergies of the chemical reactions 2 (details are givenin [1315,18]).

The model equations given in Figs. 5 and 6 aresolved numerically by computer, thereby dividingthe catalyst bed in small annular segments, typically1 mm 1 mm.

Fig. 7 compares the experimental and computed re-sults for different steam contents of fresh syngas andfor different total pressures. The agreement betweenmeasured and calculated values (axial temperatures inthe center of the reactor tube, CO-conversion) is quitesatisfactory.

2 The values of the kinetic parameters given in Fig. 6 refer only tothe temperature range where the influence of the pore diffusion onthe effective rate of reaction is fully developed, i.e. in the case ofa particle diameter of 2.5 mm, temperatures of more than 170C.External diffusion limitations can be neglected up to a temperatureof about 400C, which is a hypothetic value because the catalystalready deactivates at temperatures of more than 260C.


Fig. 6. Kinetic data of the three main reactions during FT-synthesis on the iron-catalyst (ARGE-Lurgi-Ruhrchemie).

Fig. 7 (upper part) also clearly indicates the stronginhibition of the FT-reaction (Eq. (1)) by steam,which was, in two experiments, already present in thefresh syngas and not just produced within the catalystbed. The inhibition is also reflected by the kineticequation of this reaction (Eq. (7) in Fig. 6). Along thecatalyst bed, the rate of the FT-reaction is decreasedmore and more by the steam which is formed (Eqs.(1) and (2)), and by the decreasing partial pressure ofhydrogen. After passing a maximum value of about260C at a bed length of about 0.5 m, the reactorcooling dominates the heat released by the chemicalreactions, and the axial temperatures start to decline(Fig. 7).

4. Modeling of technical FT-reactors

4.1. Aims and assumptions

Based on the experience obtained from the applica-tion of the model to the semi-technical reactors, it wasthen used to calculate the basic data of industrial mul-titubular FT-reactors. In these calculations, the pro-cess parameters (e.g. the diameter of the single tubesand the cooling temperature) were varied with respectto improving CO-conversion and the production rateof diesel oil and wax per tube. In addition, the para-metric sensitivity of the reactor was considered as thislimits the maximum diameter of the single tubes withrespect to a safe operation of the reactor.


Fig. 7. Comparison of computed and experimental results: axialtemperature profiles and CO-conversion of the first semi-technicalFT-reactor for different steam contents of syngas and for differ-ent total pressures (symbols: measured values; lines: model cal-culation).

As already mentioned, the nitrogen content of syn-gas plays a significant role in the removal of thereaction heat to the cooling medium. Therefore, inthe following, the reactor behaviour will always becompared if nitrogen-free or nitrogen-rich syngas(50 vol.% nitrogen) is converted. The latter representssyngas, which is produced by catalytic partial oxida-tion of natural gas with air, as proposed in this paperfor a low-cost process. Nitrogen-free syngas standsfor a typical syngas, which is produced by classicalprocesses like steam-refoming of natural gas, coalgasification or partial oxidation of heavy oil. 3 For thecalculations, the following test case was considered: The tubes of the FT-reactor have a length of 8 m. The fresh syngas is free from steam and the mo-

lar ratios of carbon monoxide, hydrogen, carbondioxide and methane are constant (see Fig. 8). Thesyngas has a nitrogen content of 50 vol.% or isnitrogen-free3.

3 Technical syngases of these processes always contain smallamounts of nitrogen (about 1 vol.%), which is here for simpli-fication neglected.

Fig. 8. Axial temperature profiles of a single tube of a multi-tubular FT-reactor for nitrogen-free and nitrogen-rich syngas; forcomparison, the adiabatic operation in each case is also shown(Tcool = Tin; optimal working temperature Topt = 250C to keep asafe distance to the deactivation temperature Tdeact of 260C aswell as to reach the highest possible CO-conversion).

The product gas is not recycled. This configurationis less expensive 4 because a recycle compressor isnot needed.

Unless otherwise stated, the temperature at the reac-tor inlet and the cooling temperature are the same.

The pressure (24 bar in case of 50 vol.% nitrogen) isreduced by half in the case of nitrogen-free syngasto ensure the same partial pressure of hydrogen andthus the same rate of reaction 5 and heat production,at least at the reactor entrance.

Remark: For the given reaction conditions (no re-cycle of product gas), the reduction of the total pres-sure is, in case of nitrogen-free syngas, unavoidablewith respect to a temperature runaway. Calculationswith the model clearly indicate that, for the given re-action conditions, the parametric sensitivity is oth-erwise too high in the case of nitrogen-free syngas

4 In case of N2-free syngas, a recycle compressor is normallyinstalled; for the comparison of both cases, this is not consideredhere.

5 The FT-reaction is a first-order reaction with respect to hydro-gen (see Eq. (7) in Fig. 6); the formation of methane is assumedto be a first-order reaction with respect to hydrogen (see Eq. (8)in Fig. 6).


Fig. 9. Typical radial profiles of temperature and of reaction ratein a single tube of a multitubular FT-reactor at the axial positionz (250C), where the optimal temperature of 250C is reached(computed results; conditions see Fig. 8).

(without a gas recycle), and even the smallest tubediameter taken into consideration of only 41 mmcannot be realized with respect to a runaway. (Theparametric sensitivity will be discussed in detail inSection 4.3.)

For a given tube diameter, the (standard) volumerates of carbon monoxide and hydrogen are keptconstant to ensure the same production rate ofdiesel oil and wax (in kg/s) for a given degree ofCO-conversion (see below). Consequently, the total(standard) volume rate of syngas has to be dou-bled in the case of nitrogen-rich syngas comparedto nitrogen-free syngas. This results in a constantresidence time (25 s with respect to the emptytube) in both cases (nitrogen-free and nitrogen-richsyngas) because the total pressure is, in the caseof nitrogen-rich syngas, twice as big as in case ofnitrogen-free syngas (see above).

For different tube diameters, the appropriatevolume rate of syngas is adjusted to reach aCO-conversion of about 30%. (This is equivalentto a total CO-conversion of at least about 60% inan FT-unit with two reactors arranged in series.)

The rate of CO-conversion by the FT-reaction (Eq.(1)) strongly depends on the reaction temperature:in a typical temperature range for FT-synthesis of200C up to 250C, the reaction rate is doubled incase of the iron catalyst if the temperature is in-creased by about 30 K. On the other hand, tempera-tures of more than 260C should be avoided becausethe Fe-catalyst deactivates [14], and the product dis-tribution shifts towards the production of more un-wanted methane. With respect to both a high pro-duction rate of diesel oil and wax and to a safe dis-tance of 10 K to the deactivation temperature Tdeactof 260C, an optimal working temperature Topt of250C is assumed here. So the temperature profilehas to be adjusted by the appropriate cooling tem-perature Tcool, so that Topt is just reached at thetemperature maximum.

4.2. Influence of nitrogen on CO-conversion

The axial temperature profiles both for nitrogen-richand for nitrogen-free syngas are shown in Fig. 8 fora constant tube diameter of 70 mm and the optimalworking temperature of 250C. For comparison, thetemperature profiles in adiabatic operation are alsoshown, indicating that a strong cooling and a smalltube diameter are essential for the operation of multi-tubular FT-reactors.

The Tcool is about 20C lower in the case ofnitrogen-free syngas compared to a syngas with50 vol.% nitrogen. So less carbon monoxide is con-verted in the case of nitrogen-free syngas (27% com-pared to 35% in the case of nitrogen-rich syngas)because the mean (with respect to the kinetics) axialtemperature is lower (Fig. 8). In addition, the meanradial temperature for a given axial temperature inthe center of the tube is also lower, as is shown as anexample in Fig. 9 for the respective axial positionsof the temperature maximum of 250C. The profilesof the radial temperature and of the correspondingrate of CO-conversion by the FT-reaction indicatethat the reaction rate near the wall is still about 70%of the one in the center of the tube in the case ofnitrogen-rich syngas, but only 40% in the case ofnitrogen-free syngas. (It should be noted here that thereactor is, in case of nitrogen-free syngas, almost atrunaway for the given single tube diameter of 70 mm.


Fig. 10. Parametric sensitivity of the FT-reactor towards Tcoolfor different tube diameters and nitrogen-free syngas (volume rateof syngas according to te = 25 s; other conditions as in Fig. 8;dot-dash lines: hypothetical profiles as T > Tdeact = 260C).

The temperature profile shown in the lower part ofFig. 8 is, therefore, unrealistic with respect to a safeoperation of the reactor, which will be discussed indetail in Section 4.3.)

Both radial profiles also make clear that the radialtemperature gradient within the catalyst bed can notbe neglected in modeling wall-cooled reactors with astrong exothermic reaction, i.e. that a two-dimensionalmodel, as used here, is essential.

4.3. Parametric sensitivity of the FT-reactor

Even more important than the increase in CO-conversion with increasing nitrogen content of syngasis the much lower parametric sensitivity in the case ofnitrogen-rich syngas compared to nitrogen-free syn-gas. This is illustrated in Figs. 10 and 11, which show,for both types of syngas, a typical sensitive (lowerpart of each figure) and insensitive situation (upperpart). The full lines represent the temperature profiles,where the optimal working temperature of 250C isjust reached, but not exceeded. It is important to notethat each case is attached to a different tube diameterwhich can or can not be realized without the dangerof a temperature runaway.

For nitrogen-free syngas, a tube diameter of 50 mmwould lead to a very sensitive situation (lower part ofFig. 10): if Tcool is increased by only 2 K, then themaximum temperature already exceeds the catalystsdeactivation temperature of 260C. A further increaseof Tcool by 2 K would definitely lead to a runaway.

In case of nitrogen-rich syngas (upper part of Fig.11), a much larger tube diameter of 70 mm can berealized without the danger of a temperature runaway.Increasing the diameter further to 90 mm would thenalso lead to a sensitive behavior of the reactor (lowerpart of Fig. 11).

The pronounced influence of the nitrogen contentof the syngas on the sensitivity of the multitubularFT-reactor is even more evident in the case of a con-stant tube diameter, as shown in Fig. 12 for a diam-eter of 70 mm by the plot of the maximum tempera-ture Tmax versus the cooling temperature Tcool, whichcan be taken as a measure of the response of the reac-tor (Tmax) to a small variation in a reaction parameter(Tcool).

At first, Tmax rises at about the same rate as Tcool.With increasing Tcool, the temperature curve gets apronounced curvature because the rate of reaction, andtherefore, the heat produced by the chemical reactionsincrease exponentially according to Arrheniuss law,

Fig. 11. Parametric sensitivity of the FT-reactor towards Tcoolfor different tube diameters and nitrogen-rich syngas (volume rateof syngas according to te = 25 s; other conditions as in Fig. 8;dot-dash lines: hypothetical profiles as T > Tdeact = 260C).


Fig. 12. Influence of Tcool on the maximum axial temperaturefor nitrogen-rich and nitrogen-free syngas and a tube diameter of70 mm (reaction conditions as in Fig. 8).

whereas the heat removal increases only linearly withtemperature. At a certain Tcool, a pronounced increase nearly a jump of Tmax occurs.

At this point, more heat is generated by the chemi-cal reaction than is removed by the cooling medium,and the catalyst heats up suddenly until a new sta-ble operating point is reached, at which the tempera-ture difference (Tmax Tcool) is again sufficient to re-move the generated heat from the catalyst to the cool-ing medium. This upper operating temperature is veryhigh (>1000C), as the adiabatic temperature rise isabout 760C in the case of syngas with 50 vol.% nitro-gen and 1520C for nitrogen-free syngas. So the up-per stable operating points (not shown in Fig. 12) areonly of theoretical interest because the catalyst willalready be deactivated at 260C.

As already stated before, a temperature of 250Ccan be regarded as the optimal working temperaturewith respect both to a high production rate of dieseloil and wax and to a safe distance (10 K) to the deacti-vation temperature of 260C. So the increase in Tmaxwith Tcool for Tmax = 250C can be taken as a goodmeasure for the sensitivity of the reactor if operatedunder optimal conditions.

As shown in Fig. 12 (right-hand side) for a constanttube diameter of 70 mm, the increase in Tmax withTcool (dTmax/dTcool for Tmax = Topt = 250C) is 4.5 K/Kin the case of nitrogen-rich syngas. This means thatthe deactivation temperature of 260C is not reachedunless the fluctuation of Tcool is higher than 2.2 K(=f260250Cg/4.5 K/K). This seems to be an accept-able value, which can be secured by an appropriatecontrol of Tcool.

Fig. 13. Determination of the critical working temperature with re-spect to a temperature runaway for nitrogen-rich and nitrogen-freesyngas and a tube diameter of 70 mm (reaction conditions as inFig. 8).

On the contrary, in the case of nitrogen-free syn-gas, the corresponding increase in Tmax with Tcool ismuch higher (50 K/K, see left-hand side of Fig. 12),and therefore, a fluctuation of Tcool of only 0.2 K(=f260250Cg/50 K/K) would lead to an exceedingof the deactivation temperature. So the optimal work-ing temperature of 250C can not be realized withrespect to a stable operation of the reactor, and thetemperature profile shown before in the lower part ofFig. 8 for nitrogen-free syngas and a tube diameter of70 mm is unrealistic.

To evaluate the allowable or critical working tem-perature Tcrit , which should not be exceeded to avoida temperature runaway, the following practical crite-rion was chosen, which leads to a very simple methodfor the determination of the critical temperature withrespect to a runaway: Tcrit is equal to the temperatureat which the two tangents to the lower and upper partof the curve Tmax(Tcool) intersect. As shown in Fig. 13,this definition of Tcrit is, here, equivalent to an allow-able increase in Tmax with Tcool of 5 K/K, at least forthe given reaction conditions and the kinetic parame-ters of FT-synthesis with the Fe-catalyst used here. 6

In the case of nitrogen-free syngas and a tube di-ameter of 70 mm, Tcrit is only 221C (Tcool = 180C,see left-hand side of Fig. 13), which in turn leads toa low CO-conversion of 20%. In contrast to this, asafe operation of the reactor would be possible fornitrogen-rich syngas up to a temperature of 253C if

6 According to the classical theory of ignition, the critical tem-perature difference (Tmax Tcool) is given by the expressionTmax2 EA, which is, here, about 45 K. This value is exactly reachedin both critical cases (Fig. 13).


Fig. 14. Influence of the diameter of the single tubes of aFT-reactor on the critical maximum temperature for nitrogen-richand nitrogen-free syngas (volume rate of syngas according to e = 25 s; other conditions as in Fig. 8).

the catalysts temperature would not be limited by theassumed optimal working temperature of 250C withrespect to a save distance of 10 K to the deactivationtemperature of 260C.

For a given syngas composition and a constant resi-dence time (e.g., here, 25 s), the critical working tem-perature can only be increased by decreasing the tubediameter. Further calculations for different tube diam-eters indicate that, for nitrogen-free syngas, the criti-cal tube diameter dT;crit is about 45 mm compared to73 mm for nitrogen-rich syngas, if the optimal work-ing temperature of 250C should be reached (Fig. 14).

4.4. Heat transfer parametersAt first sight, these results are striking because the

reaction conditions are the same in both cases (gas ve-locity and residence time, respectively, inlet tempera-ture, partial pressures of all reactants and thus the rateof CO-conversion at least at the reactor entrance). Thekey to understand the strong and positive influence ofthe nitrogen content of syngas on the reactor behav-ior is the role of nitrogen in the removal of the heatof reaction by convection in axial direction as well asby the radial heat transfer to the cooled wall of thereactor tubes.

Firstly, the ratio of the heat capacity of syngas tothe heat generated PnSG cp= .XCO PnCO j1RH j/ increases with increasing nitrogen content. So therise in the axial temperature in the front section of thetubes decreases with increasing nitrogen content. Forexample, the adiabatic rise in temperature

1Tad D PnCO XCOj1RH jPnSG cp 1 yN2 (10)

Fig. 15. Influence of the superficial gas velocity (with respect tostandard conditions) uS;n on the effective radial heat conductivityrad;eff and on the inner wall heat transfer coefficient W;i (re-marks: W;iand eff are within the regions discussed here practi-cally independent of the tube diameter and of the gas composition,i.e. identical for nitrogen-free and nitrogen-rich syngas; both co-efficients were calculated with the correlations given in [2023]for particles (cylinders) with a length of 5 mm and a diameterof 2.5 mm; for comparison: outer wall heat transfer coefficientW;o = 365 W/(m2 K).

is only about 7 K for a CO-conversion XCO of 1% com-pared to 14 K in case of a nitrogen-free syngas. Corre-spondingly, the fall in temperature in the rear sectionof the tubes is reduced in the case of nitrogen-rich syn-gas. So the axial temperature profile gets more shallowwith increasing nitrogen content, and the mean (withrespect to the kinetics) axial temperature and thus theCO-conversion for a given limit of the temperature(here, 250C) increase.

Secondly this effect is even more important thanthe one described above rad;eff increases with in-creasing nitrogen content of syngas if like in thepresent case the standard volume rates of carbonmonoxide and hydrogen are kept constant: rad;eff isproportional to the superficial gas velocity 7 uS as wellas to the molar density ct and to the molar specificheat cp of syngas [16,20]:rad;eff cp ct uS (11)Rewriting of Eq. (11) leads to

rad;eff cp ct;n Pvn;SGd2T

(12)

So for a given diameter of the tube dT, rad;eff onlydepends on the total standard volume rate of syngas as

7 The static contribution (uS = 0) to rad;eff is very small(0.21 W/(m K) [16]; see Fig. 15) and is neglected in the Eqs. (11)and (12)).


Fig. 16. Parametric sensitivity of the FT-reactor towards Tcoolfor a nitrogen-free syngas at a relative high feed rate of syngasof 85 m3 syngas/h (STP) (other reaction conditions as in Fig. 8;dot-dash line: hypothetical profile as T > Tdeact = 260C).

the molar specific heat of syngas, as well as, the molardensity with respect to standard conditions can be con-sidered to be independent of the nitrogen content. Incase of the proposed process with nitrogen-rich syngas(about 50 vol.% nitrogen), the standard volume rate ofsyngas and the gas velocity with respect to standardconditions are doubled compared to nitrogen-free syn-gas in order to adjust the same standard volume ratesof carbon monoxide and hydrogen. Therefore rad;effis also doubled (Fig. 15), which helps to transfer thereaction heat from the catalyst to the wall and thusto remove it by the cooling medium. Similar con-siderations are true for the inner wall heat transferco-efficient W;i, which also increases with increas-ing nitrogen content, although not as pronounced asrad;eff (Fig. 15).

Consequently, a tube diameter of more than 45 mmcan only be realized in the case of nitrogen-free syn-gas by a much higher gas velocity and total volumerate of syngas. As shown in Fig. 16, a diameter of70 mm is only possible in the case of nitrogen-freesyngas if the total standard volume rate of syngas isincreased from 28.5 to 85 m3/h ( e decreases 25 to8.4 s). rad;eff as well as W;i increase, and in addi-tion, the heat generated per length of the reactor de-creases because of the shorter residence time, whichdecreases the CO-conversion from 36% (see Fig. 10,dT = 41 mm) down to only 15%.

Similar considerations are true for a smaller tubediameter but a higher total pressure. For example, themultitubular FT-reactors, which are in operation inSouth Africa, work (with respect to a nitrogen-freesyngas) at a relatively high total pressure of 24 bar and

with a diameter of the singe tubes of 46 mm. In orderto assure a high gas velocity, which is needed for thesafe operation of the reactor, fresh syngas is mixedwith recycle gas in a ratio of 1 : 2. In addition, thepartial pressure of hydrogen is thereby reduced (Fig.17), which also decreases the danger of a runaway.Although the tubes are 12 m long, the CO-conversionis about the same as in case of a nitrogen-rich syngasand a tube length of 8 m (Fig. 17, see also the upperpart of Fig. 8). The calculated critical working tem-perature of the ARGE-reactor is exactly equivalent tothe maximum temperature of the process of 250C[24] (Fig. 17), and a further increase in the diameteris impossible without the danger of a runaway.

The calculated data of the ARGE-process under-line, that the reactor model is quiet accurate: the cal-culated CO-conversion is 34% with respect to the to-tal feed gas, and 82% with respect to the fresh feedgas, which is in good agreement with data reported inthe literature (31% and 73%, respectively), especiallyif one considers that the process data were taken fromdifferent literature [2426].

Fig. 17. Data of the technical fixed-bed multitubular ARGE-reactoras operated in South Africa (data were calculated with the reactormodel; reaction conditions as given in [2426]; the ARGE-reactoris equipped with 2052 single tubes with an inner diameter of46 mm).


Fig. 18. Influence of the tube diameter on the production rate ofdiesel oil and wax per tube (volume rate of syngas according to e = 25 s; other conditions as in Fig. 8).

4.5. Production rate of diesel oil and wax

The strong influence of the tube diameter as well asof the nitrogen content of syngas on the production rateof diesel oil and wax per tube Pmcond is shown in Fig.18. Due to the effects described before reduction ofthe mean (with respect to the kinetics) axial as well asof the radial temperatures and thus of CO-conversionwith increasing diameter Pmcond is proportional tod1:4T (nitrogen-free syngas) and to d1:7T (nitrogen-richsyngas), respectively, and not to d2T, as expected, ifonly the alteration of the cross-section of the tubeswould have to be considered 8 . Correspondingly, thetotal number of tubes for a given production rate ofdiesel oil and wax is inversely proportional to d1:4T andto d1:7T .

If, in addition, the parametric sensitivity is consid-ered, then the production rate of diesel oil and waxper tube is, in the case of nitrogen-rich syngas, higherby a factor of about 3 than in the case of nitrogen-freesyngas because the tube diameter is limited in the caseof nitrogen-free syngas to 45 mm, whereas for syngas

8 If the CO-conversion is constant, then Pmcond per tube is propor-tional to the volume rate of syngas, and therefore, for a constantgas velocity and pressure, proportional to d2T.

with 50 vol.% nitrogen, the tube diameter can be ex-tended to 70 mm (Fig. 18).

As the dilution of syngas with nitrogen very ef-fectively helps to remove the heat generated by theFT-reaction, the total number of tubes can be signifi-cantly reduced, whereas for a given production rate ofdiesel oil and wax, the total mass of catalyst is almostthe same.

Nevertheless, up to now, only nitrogen-free syngasis used in industrial practice (e.g. in the ARGE-processin South Africa). Therefore, the diameter of the reactortubes is smaller (46 mm), and in addition, a recycleloop of syngas is installed to increase the gas velocityand to decrease the partial pressure of hydrogen.

If nitrogen-rich syngas is converted, like in the pro-posed process, the diameter of the tubes can be in-creased, e.g. in the present case, up to about 70 mm,which will very effectively increase the production rateof diesel oil and wax per tube, and therefore, decreasethe investment costs (manufacturing costs of the mul-titubular FT-reactors).

5. Technical FT-process based on nitrogen-richsyngas

For the calculation of the basic data of a technicalprocess for the conversion of natural gas to diesel oiland wax according to the proposed low-cost processbased on nitrogen-rich syngas (Fig. 19), the followingassumptions were made: The steam content of syngas entering the first mul-

titubular reactor is 5 vol.%, which corresponds tosyngas generated by catalytic partial oxidation ofnatural gas with air [11,12,16,17].

Syngas is converted in two FT-reactors arrangedin series. The single tubes of both reactors have adiameter of 70 mm and a length of 8 m.

The product gas of the first reactor is cooled, e.g. to30C, and the water, which inhibits FT-synthesis,is separated. The feed gas of the second reactor is,therefore, practically dry.

The gas velocity in the first reactor is limited corre-sponding to a maximum pressure loss of 2 bar; thepressure loss of the second reactor is then about thesame.

The temperature should not exceed 250C (safe dis-tance to the deactivation temperature of 260C),


Fig. 19. Flow sheet of the proposed low-cost process for the conversion of natural gas to diesel oil and wax based on nitrogen-rich syngas.

which is ensured by adjusting the respective Tcool.For the optimal design of both multitubular

FT-reactors, three questions still have to be treatedwith respect to maximize the CO-conversion and theproduction rate of diesel oil and wax per tube:1. What is the optimal total pressure, as up to

now, a pressure of 24 bar was assumed for thenitrogen-rich syngas?

2. What is the influence of the steam content of syn-gas on the performance of the first FT-reactor, asin the cases discussed before, only dry syngas wasconsidered, and syngas of the proposed low-costprocess, which is produced by catalytic partial ox-idation of natural gas with air, has a steam contentof about 5 vol.%?

3. What are the optimal inlet and cooling tempera-tures? As of now, both temperatures have alwaysbeen set equal.

The answers are given in Figs. 20, 21 and 22,thereby assuming in each case that the optimal work-ing temperature of 250C is reached:1. As shown in Fig. 20, the total pressure should be

less than about 26 bar because at higher pressures,the danger of a runaway occurs. So a pressure of24 bar, as assumed before for the calculations, is agood value with respect to a high production rateas well as to a safe operation of the reactor.

Fig. 20. Influence of the total pressure on the production rate ofdiesel oil and wax per tube for nitrogen-rich syngas and a tubediameter of 70 mm (first multitubular FT-reactor; volume rate ofsyngas according to a pressure loss of 2 bar, i.e. 33 m3/h (STP) at8 bar and 74 m3/h (STP) at 40 bar; Tcool = Tin; Tmax = Topt = 250C;syngas without steam as in Fig. 8).

2. Steam inhibits FT-synthesis (see e.g. the upperpart of Fig. 7). Therefore, the critical tube diam-eter increases slightly with increasing steam con-tent of the fresh syngas (Fig. 21). Nevertheless,for the design of the FT-reactors of the proposedprocess, a tube diameter of 70 mm was still cho-sen to be far away enough from the regime wherea temperature runaway could occur, and to avoidan extrapolation from experimentally proved data,which is too far away from the diameter of thesemi-technical reactors (41 mm). Further experi-


Fig. 21. Influence of the steam content of syngas on the criticaltube diameter with respect to a temperature runaway of the firstmultitubular FT-reactor (Tcool = Tin; Tmax = Topt = 250C; volumerate of syngas according to te = 25 s; pt = 24 bar (nitrogen-richsyngas) and 12 bar (nitrogen-free syngas); molar ratios of syngascomponents as in Fig. 8).

Fig. 22. Influence of the inlet temperature of syngas on the per-formance of the first multitubular reactor of the proposed low-costprocess for the conversion of nitrogen-rich syngas to diesel oil andwax (dT = 70 mm; Tmax = Topt = 250C; other reaction conditionsas in Table 2).

ments should, therefore, have to be done to provewhether it is possible to extend the diameter to80 mm, which would increase the production ofdiesel oil and wax per tube by a factor of 1.3. Fig.21 also indicates that in the case of a nitrogen-freesyngas, the diameter could only be extendedfor a syngas with 5 vol.% steam, and for thegiven reaction conditions, from 45 mm to about50 mm.

3. The influence of the inlet temperature of syngason CO-conversion is shown in Fig. 22 for thefirst multitubular FT-reactor. Tcool was simulta-neously changed until, for each inlet temperature,the optimal working temperature of 250C wasreached.

For the given reaction conditions (as listed in Table2), the CO-conversion as well as Tcool only slightlyincrease up to an inlet temperature of 230C. A fur-ther increase in the inlet temperature then results ina pronounced decline of both CO-conversion andTcool. 9 So for the first FT-reactor, the optimal inletand cooling-temperature, respectively, are 230 and216C. A similar calculation was done for the secondreactor.

The optimal design parameters of the two multi-tubular FT-reactors arranged in series according tothe criteria as stated above are listed in Table 2. Therespective temperature profiles of both reactors areshown in Fig. 23. The total CO-conversion reachedafter passing both reactors arranged in series is 63%.The corresponding basic data of an industrial plant(Fig. 19) for the annual production of about 500,000 tdiesel oil and wax are also listed in Table 2, indicatingthat 16 multitubular FT-reactors are needed, equiva-lent to 8 parallel production lines with two reactors inseries. Each reactor is equipped with 4000 tubes witha length of 8 m and a diameter of 70 mm, and the to-tal number of tubes of the FT-unit is 64,000. For anevaluation of this result, two cases should be finallymentioned: In case of the classical tube diameter of 46 mm,

the total number of tubes would be about twotimes higher than in the proposed low-cost processwith a diameter of 70 mm (see Fig. 18, number oftubes (1/dT)1:7). This underlines the significantinfluence of the tube diameter on the total numberof tubes and on the manufacturing costs of theFT-reactors.

In the multitubular FT-reactors of the ARGE-pro-cess in South Africa, 16,000 t per annum of valuableproducts (mainly diesel oil and wax) are producedper reactor, which is equipped with 2052 tubes of12 m length and an inner diameter of 46 mm. For the

9 In addition, the lowest cooling temperature with respect to arunaway is about 205C (for the given maximum temperature of250C), see footnote 6.


Table 2Basic data of an industrial plant of the proposed low-cost process for the conversion of natural gas to diesel oil and wax on the basis ofnitrogen-rich syngas with about 50 vol.% nitrogen (availability of the plant: 8000 h per annum)Synthesis gas generation

Volume rate of natural gas (m3 per annum) (STP) 2.53 109 (1.35 106 tC per annum) (Hu = 36 MJ/m3)Volume rate of air (m3 per annum) (STP) 9.3 109Volume rate of fresh syngas (composition see Fig. 4), 14.6 109

(m3 per annum) (STP)Total pressure (bar) 26Diameter of syngas reactor (catalyst bed) (m) 3Number of syngas reactors (calculated with data from [11,12]) 4FischerTropsch synthesis 1. Synthesis step 2. Synthesis step

Total pressure (reactor inlet)a (bar) 25 23Cooling temperature (C) 216 215Temperature at the reactor inlet (C) 230 180Length of reactor tubes (m) 8 8Diameter of reactor tubes (inner) (mm) 70 70Total number of tubes 32,000 32,000Number of FT-reactors (4000 tubes/reactor) 8 8Diameter of reactor (inner)b (m) 7.5 7.5CO-conversion (related to CO in the syngas of 36 42

each reactor) (%)Production rate of diesel oil and wax (t per annum) 248,000c 236,000cTotal production rate of diesel oil and wax (t per annum) 484,000 (415,000

tC per annum) (XCO;t = 63%)Volume rate of off-gas (m3 per annum) (STP) 10.7 109Heating value Hu of off-gasd (MJ/m3) (STP) 3.5a The pressure loss is as of now not considered in the model, and is calculated separately. For the calculations, a mean total pressure(=(Pin + Pout)/2) of 24 bar (reactor 1) and 22 bar (reactor 2) was assumed.b Calculated with data from [27], thereby assuming an interval between the tubes of 30 mm.c The selectivity to CO2, which is formed by the shift reaction (see Eq. (3) and Eq. (9)), is higher in the first reactor than in the secondone because steam is already present in the syngas of the first reactor, whereas the syngas of the second reactor is dry. The productionrate of diesel oil and wax is, therefore, not exactly proportional to the CO-conversion, which is reached in each reactor.d Typical off-gas composition: 64 vol.% N2, 21 vol.% H2, 8 vol.% CO, 5.5 vol.% CO2, 1.5 vol.% CH4, 0.4 vol.% C2C10 (calculated asC3H8)[16,17].

production rate, which is assumed here (484,000 t ofdiesel oil and wax per annum, see Table 2), 62,000single tubes of 12 m length would be needed com-pared to 64,000 single tubes of 8 m length in theproposed process. So about the same productionrate per tube is reached in the proposed processwith nitrogen-rich syngas, but without any syngasrecycle, and without a costly syngas generation bysteam reforming or by partial oxidation with oxy-gen, which requires an air separation plant.

6. Conclusions and outlook

The economic utilization of natural gas resourcesin remote areas is limited by its costly transportation.A solution of this problem could be the conversion to

Fig. 23. Axial temperature profiles of the single tubes of the firstand second multitubular FT-reactor of the proposed low-costprocess with nitrogen-rich syngas for the conversion of naturalgas to higher hydrocarbons (reaction conditions as in Table 2).

higher hydrocarbons by the following low-cost pro-cess: after desulphurization, natural gas is convertedinto a nitrogen- rich and soot-free syngas by catalytic


partial oxidation with air. This syngas is then con-verted by FT-synthesis into diesel oil and wax in twofixed bed multitubular reactors in series.

This concept was successfully tested in a semi-tech-nical plant. For the experiments, FT-reactors with adiameter corresponding to a single tube of commonindustrial multitubular reactors (41 mm) were used.

A mathematical model for the FT-reactor has beendeveloped, which is based on a set of equations thatencounter the heat and mass transfer and the reactionkinetics.

Calculations with this model indicate that under thespecific conditions of the proposed low-cost processwith nitrogen-rich syngas (about 50 vol.% nitrogen)a diameter of the reactor tubes of 70 mm can be re-alized without the danger of a temperature runawaybecause nitrogen helps very effectively to remove thelarge amounts of heat generated by the FT-reaction.In the case of nitrogen-free syngas, the tube diameteris limited to about 45 mm to avoid a runaway if no re-cycle of syngas is installed. Thereby, the productionrate of diesel oil and wax per tube is comparedto nitrogen-rich syngas reduced by a factor of 3,which underlines the advantage of the low-cost con-cept based on nitrogen-rich syngas: the increase in thetube diameter leads to a drastic reduction in the totalnumber of tubes and thus to a decrease in the invest-ment costs of an industrial FT-plant (manufacturingcosts of the multitubular FT-reactors) for the conver-sion of natural gas to diesel oil and wax.

Detailed economic studies are still necessary to val-idate or disprove whether and under which circum-stances the proposed process with nitrogen-rich syn-gas is an attractive alternative to classical processeswith nitrogen-free syngas (like the SMDS-process ofShell), especially in areas with remote natural gas re-sources. The effect of off-gas usefulness and valueshould also be included. In addition, the compressioncosts, which are higher in the case of an air compres-sor (compared to oxygen) should be considered. Nev-ertheless, the results presented in this paper indicatethat it is promising to go ahead with further model cal-culations as well as with experimental investigationsin order to improve the efficiency of the FT-reactors(output of diesel oil and wax per tube), especially withregard to the following subjects: The model calculations, which indicate an allow-

able tube diameter of 70 mm for nitrogen-rich syn-

gas (and a feasible extension to 80 mm, see Fig.21), should be proven by experiments with singletube reactors of larger diameters (as up to now,dT = 41 mm).

The model for the simulation of the FT-reactoris based on the assumptions of constant porosity,constant effective radial heat conductivity and plugflow. The radial heat transfer near the wall is evalu-ated from a postulated near-wall temperature jumpin combination with W;i, which is then needed toaccount for the particular conditions of heat transfernear the wall (near-wall flow channelling, near-wallporosity and velocity changes).According to the literature, e.g. [28,29], the pre-

dictions of temperature profiles and hot spots ofwall-cooled fixed bed reactors can be improved byincorporating velocity and porosity profiles, espe-cially in the region of smaller Reynolds numbers Repup to about 400 10 . The radial heat transfer near thewall (typically within a distance of 2dp) can then alternatively to the concept used here be describedby a locally varying rad;eff (r), and so the introduc-tion of W;i can be omitted. It would be of interest toinvestigate whether or not a refinement of the presentreactor model is justified. The mean chain length, and therefore, the hydrocar-

bon distribution of FT-synthesis changes with tem-perature. At higher temperatures, the mean chainlength is smaller, and more methane and lighter hy-drocarbons and less diesel oil and wax are formed.Nevertheless, the influence of the reaction temper-ature on the product distribution (see [1315]) is,of now, not considered in the model used here. Theresults presented in this paper were calculated withthe hydrocarbon distribution, which was obtained inthe semi-technical plant (Fig. 4), and which, there-fore, only represents a mean distribution in the tem-perature range of 225260C. For an accurate deter-mination of the amounts of different product frac-tions like diesel oil, wax, lighter hydrocarbons, andmethane, the influence of the temperature on theprobability of chain growth should be introducedinto the model.

10 For comparison: for typical conditions (dp = 2.5 mm; mean tem-perature = 240C, uS = 0.3 m/s), Rep is 365 for syngas with 50 vol.%nitrogen (p = 24 bar) and 125 for nitrogen-free syngas (p = 12 bar).


In addition, the model for the reactor simulationshould be extended and improved by including thedynamic behaviour of the reactor, e.g. to investigatethe start-up or shut-down behaviour of the reactor.

The temperature control of the FT-reactors can beimproved by a defined change of the overall heattransfer coefficient along the reactor tubes corre-sponding to the respective heat generated, either bya defined insulation of the rear section of the tubesor by a dilution of the catalyst in the front section(see Fig. 23). Both options will probably lead to amore well-balanced axial temperature profile, andthus, to a higher mean (with respect to reaction ki-netics) axial temperature, and therefore, to a higherCO-conversion.

Other catalysts should be tested, e.g. such as thosebased on cobalt, which have, according to the liter-ature (e.g. [30,31]), a very low CO2-selectivity ofless than 1 C% compared to about 25 C% in case ofthe Fe-catalyst (Fig. 4).At the end, economic evaluations based on these

data should be done in order to develop an econom-ical process for the production of diesel oil and waxfrom natural gas in areas which are remote from themarket.

7. Nomenclature

7.1. Symbols and abbreviations

ci concentration (i: CO, H2, H2O) (mol/m3)ct total concentration of syngas (mol/m3)cp specific heat of syngas (J/(mol K))dP particle diameter (m)dT (inner tube) diameter (m)dT;crit critical (inner) tube diameter with respect to

a temperature runaway (for Tmax = Topt) (m)EA activation energy (J/mol)FT FischerTropschHu net calorific value (J/m3)kFT rate constant of FT-reaction (Eq. (1))

(m3/(kgcat s))kM rate constant of methanation reaction (Eq. (2))

(m3/(kgcat s))kS rate constant of shift reaction (Eq. (3))

(m3/(kgcat s)

k0;FT pre-exponential factor of the FT-reaction(Eq. (1)) (m3/(kgcat s))

k0;M pre-exponential factor of methanation reac-tion (Eq. (2)) (m3/(kgcat s))

k0;S pre-exponential factor of shift reaction (Eq.(3)) (m3/(kgcat s))

L length of the FT-reactor (catalyst bed) (m)mcat mass of catalyst (kg)_mcond production rate of organic condensates (diesel

oil and wax) (kg/s)_nco molar flux of carbon monoxide (mol/s)_nSG molar flux of syngas (mol/s)pt total pressure (Pa)r radial coordinate (reactor axis at r = 0) (m)rCO;i reaction rate of CO-conversion by the reaction

i (molCO/(kgcat s))rCO;FT reaction rate of CO-conversion by the FT-

reaction (Eq. (1)) (molCO/(kgcat s))rCO;M reaction rate of CO-conversion by the metha-

nation reaction (Eq. (2)) (molCO/(kgcat s))rCO;S reaction rate of CO-conversion by the shift

reaction (Eq. (3)) (molCO/(kgcat s))R universal gas constant (8.314 J/(mol K))Ri inner radius (of the tube of the FT reactor)

(m)Ro outer radius (of the tube of the FT reactor)

(m)Rep Reynolds number (uS dp/n) (dimensionless)STP standard conditions (1.013 bar, 0C)T reaction temperature (C, K)Tcool temperature of cooling medium (C, K)Tcrit critical working temperature with respect to a

temperature runaway (C, K)Tdeact deactivation temperature of the Fe-

catalyst = 260CTmax maximum temperature (C, K)Topt optimal working temperature (250C) with re-

spect both to a safe distance to Tdeact and tothe highest possible degree of CO-conversion(C, K)

uS superficial gas velocity (m/s)Pv volume rate (m3/s)XCO;R1 degree of CO-conversion in the first (techni-

cal) FT-reactor (dimensionless)XCO;R2 degree of CO-conversion in the second (tech-

nical) FT-reactor (with respect to the CO atthe reactor inlet) (dimensionless)


XCO;t degree of total CO-conversion (reactor 1 and2) (dimensionless)

yH2O molar content of steam (dimensionless)yN2 molar content of nitrogen (dimensionless)z axial coordinate (reactor entrance at z = 0) (m)

7.2. Greek letters

W;i inner wall heat transfer coefficient(W/(m2 K))

W;o outer wall heat transfer coefficient(W/(m2 K))

1RH enthalpy of reaction (J/mol)1RH298 standard enthalpy of reaction (1.013 bar,

25C) (J/mol)1Tad adiabatic temperature rise (K)W heat conductivity of the wall (W/(m K))rad;eff effective radial heat conductivity

(W/(m K)) kinematic viscosity (m2/s)B catalyst bulk density (kg/m3) e residence time with respect to the empty

reactor (s)

7.3. Subscripts (if not already listed above)

in inlet (of reactor)n standard conditions (1.013 bar, 0C)

Acknowledgements

Financial support by the Federal Ministry of Re-search and Technology of Germany is gratefully ac-knowledged (0326563A). The authors also wish tothank Ruhrchemie AG (now Celanese Inc., HoechstAG) for supplying the catalyst.

References

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Jess 1999

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