A NOVEL ROUTE FOR CONVERTING AROMATICS INTO HYDROGEN VIA STEAM REFORMING.

395BLOCK 2 — FORUM 11

A NOVEL ROUTE FOR CONVERTING AROMATICS INTO HYDROGEN VIA STEAM REFORMING.

F. Fuder corresponding author; Aral Forschung, D–44789 Bochum, Germany

D. Landwehr, G. Geipel, C. Herkt-Bruns

Degussa AG, Creavis Technology & Innovation, D–45764 Marl, Germany

J. Weitkamp Institute of Chemical Technology, University of Stuttgart, D–70550 Stuttgart, Germany

Abstract The hydrocarbons which are easy to reform into hydrogen are hydrocarbons with a chain length of less

than six carbon atoms. A new catalyst system which was jointly developed by the group at the University

of Stuttgart and Veba Oel opens up a novel route for production of mainly a n-paraffin feedstock with a

chain length of less than six carbon atoms from a feed with a high aromatic content.

The ratio between aromatic and paraffin compounds in the feed, controls the outlet temperature of the

reactor. The aromatics are converted by hydrocracking. The enthalpy of this step is only used as the

energy source, for the hydrocracking process including the evaporation of the feed. The aromatics which

tend to create problems in the usual steam reforming reaction, are the energy source in this process for

the evaporation of the liquid fuel. The crack reactor does not need any external heat and the outlet

temperature at around 400ºC is controlled by the ratio between aromatic and paraffin compounds in the

fuel itself.

The hydrocrack product can now be easily converted into hydrogen by the use of a membrane reactor

concept. The application of high pressure and a high hydrogen partial pressure enables the use of

membranes with a Pd layer of less than 10 µm thick. This membrane was developed by our partner

Creavis. In comparison to conventional membrane process, this new route provides an economical benefit

in terms of Pd content and exchange area. The pressure for the process is mainly produced by liquid

pumps, this is also a great advantage in comparison to conventional partial oxidation for fuel cell systems.

Introduction With the advent of the European Auto Oil Programme in 2005, the content of aromatics in

gasoline will have to be reduced from 42 vol.-% to less than 35 vol.-%. Comparable or even more

stringent legislation exists or will be implemented in other parts of the world, for example in

California the aromatics content of gasoline is limited to 22 vol.-%. The problem with aromatics in

gasoline is that the exhaust gas of internal combustion engines may contain small amounts of

aromatics. On the other hand, the high octane numbers of aromatics are very beneficial. Using

other components as octane boosters, e.g., methyl-tert.-butyl ether (MTBE), brings about other

problems. A potential way out of all these problems is a switch to diesel engines which, however,

inevitably leads into the problem of particulate emission. The ultimate solution for all these

environmental problems is often seen in the fuel cell technology.

Upon limiting the aromatics content of gasoline and introducing the fuel cell technology, two major

challenges will have to be mastered, viz.

– finding an outlet for the surplus aromatics and

A NOVEL ROUTE FOR CONVERTING AROMATICSINTO HYDROGEN VIA STEAM REFORMING

396 BLOCK 2 — FORUM 11

– producing hydrogen in a straightforward and economic manner from today’s fuels.

Most fuel cell systems require hydrogen as an energy source, hence large efforts are currently

undertaken to make hydrogen available in cars. Among the routes discussed are steam reforming

and partial oxidation of conventional hydrocarbon fuels or the use of alternative liquid fuels, such as

methanol or liquefied hydrogen. The latter solutions would call for a completely new infrastructure

for fuel supply and/or a new technology for producing the large amounts of hydrogen. Obviously,

the introduction of such non-conventional fuels would require huge investments by the pertinent

industry.

Hydrogen as an automotive fuel could either be manufactured from conventional hydrocarbon

sources in refinery-like stationary plants or on board the cars by direct reforming. Arguments in

favor of the second alternative include the availability of a well established distribution network for

hydrocarbon fuels, the high energy density of the hydrocarbons to be transported as compared to

hydrogen and the low technical risk of hydrocarbon versus hydrogen transportation. We propose

here a novel route for the on-board conversion of aromatics-containing hydrocarbon fuels into

hydrogen.

Description of the Reaction System

The novel route for converting aromatics-containing hydrocarbon fuels into hydrogen takes

advantage of two effects: First, the hydrogenation of aromatics is highly exothermic, and use can be

made of this heat for the vaporization of the liquid feed and for its preheating to the entrance

temperature into the steam reforming reactor. Second, the application of high pressure for the

separation of hydrogen by means of a membrane enables at the same time the separation of

carbon dioxide via its liquefaction. This latter feature allows the recovery of an off-gas with a high

heating value from the membrane reactor. As shown in Figure 1, the process consists of two main

reaction systems referred to as the pretreatment step and the steam reforming step (Figure 1). The

steam reforming step is carried out in a membrane reactor.

Figure 1: Schematic of the reaction system


397FORUM 11 — THE FUTURE OF AROMATICS IN MOTOR FUELS ANDITS IMPACT ON REFINING AND PETROCHEMISTRY

The Pretreatment Step (Ring Opening Reaction)

VEBA OEL AG and the Institute of Chemical Technology, University of Stuttgart, jointly developed

a new catalyst which allows the conversion of aromatic hydrocarbons with hydrogen into C2+-n-

alkanes consisting mainly of ethane, propane and n-butane1,2,3. These hydrogen-rich, light paraffins

can easily be converted into hydrogen via steam reforming. Table 1 shows results of experiments in

which a toluene/n-decane model mixture was catalytically converted in an autothermal fixed-bed

reactor at a hydrogen pressure of 40 bar.

Table 1: Results of the cracking of toluene and decane with hydrogen, autotherm.

The ratio between these two compounds is so that the reaction produces enough energy to heat

the gas up to 400ºC. The results are shown in table 2. We measured the heating consumption of

the reactor system to show this effect. The temperature is independent from the amount of fuel

which goes through the reactor system. The systems work autotherm. The line with base load

means the energy consumption without reaction. This system evaporates the fuel which is called

“Chemical Evaporator” it can also evaporate commercially available liquid fuels. Therefore, if

the content of aromatic compounds is too low, the reactor must be heated. The advantage of the

aromatics is, that the reactor needs only a small electric heating system and this makes the whole

system smaller because the reactor needs no combusting heating system like a gas burner. One of

the big tasks is, in the whole system is hydrogen and after the Pre-treatment step won’t be aromatic

molecule in the stream, so there no cooking effects in the system.

Table 2: Energy consumption during the crack reaction

To find a solution for the aromatic rich pyrolysis gasoline by naphtha steamcracking, we developed

a new process together with Linde and Sued Chemie. This commercial process presently available

convert pyrolysis gasoline into a high-value synthetic steamcracker feedstock. This process is called

ARINO (Aromatics Ring Opening) process. The commercial catalyst therefore was developed by

Sued Chemie.

WHSV [1/ h] 0.6 1.0 2.0Temperature [°C] 381 370 312Methane [vol.%] 1.9 3.4 0.2Ethane [vol.%] 2.8 5.6 1.0Propane [vol.%] 78.2 41.6 79.7Butane [vol.%] 11.8 32.4 8.7Residual>C4 [vol.%] 5.2 17.0 10.4

WHSV [1/h] 0.49 0.24 0.73 0.49Temperature [°C] 343 338 345 343Toluene/n-Decane [g/h] 157 78 237 158Hydrogen [g/h] 13.2 6.7 20 13.2Power [W] 82 109 88 89Base load [W] 126 133 126 126



The Steam Reforming Step

The gas consisting of C2+-n-alkanes leaves the pretreatment step with a temperature of about

400ºC. In a separate water evaporator, water is vaporized and heated to the same temperature.

Both gas streams are above their critical temperatures, so their mixing will be free from problems. A

conventional steam reforming catalyst is used in the reactor, the active component being nickel or a

noble metal, depending on the sulphur content of the feedstock. The reaction temperature

required in the steam reforming reactor is well above 500ºC.

The thermodynamics of the steam reforming reaction are strongly influenced by the pressure, as

shown in Figure 2. It is evident that methane formation is favored at high pressures.

Figure 2: Thermodynamic concentration as function of the pressure (Propane:Water=1:7@750ºC).

For economic reasons, the membrane reactor must be operated at high pressure, the ultimate

reason being the high cost of palladium. As shown in Figure 3, the economic requirements can be

met at a membrane thickness of about 10 µm and a pressure above 40 bar. The data in Figure 3

were calculated on the basis of a palladium price of 14 US $/g, a PEM efficiency of 50% and

experimentally determined permeation rates of hydrogen through the membrane.



Figure 3: Costs for the palladium per kW of installed electrical power of the PEM.

Among the main advantages of a membrane reactor is the possibility to exceed the thermodynamic

yield limitations. This allows to operate the steam reformer at a high pressure. As shown in Figure 2,

higher pressures result in decreased CO concentrations, i.e., the concentration of CO2 is higher

which is important for the separation unit. A very important point is the quality of the hydrogen

produced. The hydrogen from the membrane reactor can be directly used in a fuel cell, i.e., no

additional steps like sulphur removal or treatment of carbon monoxide are required. Furthermore,

a lower surface area of the fuel cell can be employed, since there is no nitrogen which must

normally pass through the fuel cell. The off-gas from the membrane reactor consists of carbon

dioxide, methane, carbon monoxide, water vapor and hydrogen, the concentration of the latter

depending mainly on the membrane area and the reaction temperature. An open reactor with the

membrane inside is shown in Figure 4.

The off gas separation step.

The are two possibilities to produce a gas with a high heating value from the membrane reactor off-

gas. Firstly, carbon dioxide can be directly liquefied at high pressure, whereby a heating gas is

simultaneously produced. Secondly, upon passing the gas through a nozzle, carbon dioxide partly

solidifies, which brings about a separation as well. It is noteworthy that the off-gas from the

membrane reactor itself has a higher heating value than the off-gas from a normal partial oxidation,

because the former is not diluted with nitrogen. The off-gas from the membrane reactor can hence

directly be used to heat the steam reformer.

Since the off-gas from the membrane reactor serves as the energy source for the steam reformer,

methane is needed as a component due to its high heating value. This is why the higher yields of

methane due to the higher pressure are not a disadvantage. In other words maximizing the

conversion to hydrogen is not aimed at, instead the conversion has to be optimized such as to leave



enough heating value in the off-gas. This is an essential point which makes the system control easier

and reduces the amount of catalyst and the membrane area required.

Figure 4: View of a palladium membrane reactor.

The membrane manufacturing process.

Based on woven stainless steel with a first Al2O3 layer microfiltration membranes could be

achieved. By further coating with nanoscale metal oxide particles smaller pore sizes for ultra- and

nanofiltration applications are realized. Thus membrane foils, less than 80 micrometers thick, are

produced in a continuous process, realizing variable pore sizes down to small nanometer scale. The

degree of chemical and temperature stability required determines which material is used for the

support.

The Ceramic foil production is a continuous process. The production unit coats the woven fabric

with the inorganic suspension, the fabric is dried and hardened. A visual detector excludes defective

material. After the ceramic coating process a PVD-process is attached for finishing the membrane

with a palladium coating. In dependence of the process parameters thickness of the palladium layer

is controlled.

The structure of the hydrogen separation membrane is shown in Figure 5. The TEM detail displays

the surface of the coated membrane.



Figure 5: Hydrogen separation membrane

The blending of aromatic and par-affin compounds.

To determine the ratio of aromatic and paraffinic hydrocarbons to be blended, it is useful to classify

their carbon atoms into various groups. The main criterion is the number of hydrogen atoms bound

to the carbon atoms. The ratio between aromatic and paraffin components determined the outlet

temperature of the reactor.

Table 3: Structure parameter and model parameter for blending purpose.

Nanofiltration: Pores 0.5–5nm

Ultrafiltration: Pores 5–50 nm

Microfiltration: Pores 50–500 nm

80 µm

Catalytic membranes

Woven Fabric Support

Parameter [kJ/mol] 26.41 4.95 -9.59 -29.12 -39.78Component Paraffin Paraffin Paraffin Aromatic Aromatic Reaction Model

CH3 CH2 CH CH C Enthalpy Enthalpy[kJ/mol]

n-Decane 2 8 0 0 0 92 92n-Octane 2 6 0 0 0 82 82Ethylcyclohexane 1 6 1 0 0 45 46Ethylbenzene 1 1 0 5 1 -155 -154o-Xylene 2 0 0 4 2 -143 -143n-Heptane 2 5 0 0 0 78 78Methylcyclohexane 1 5 1 0 0 43 42Toluene 1 0 0 5 1 -158 -159Benzene 0 0 0 6 0 -175 -175n-Hexane 2 4 0 0 0 73 73Cyclohexane 0 6 0 0 0 31 30



Table 4: Example mixture for testing purpose.

Conclusion The system described in this paper offers the opportunity to produce hydrogen on a small scale. It

does not require complex gas treatment systems, and it can be operated with proven state-of-the-

art catalysts in a conventional shape, e.g., pellets. The pressure can be generated by means of liquid

pumps which is a significant economical advantage. Just the hydrogen needs a gas compressor, but

only 10 to 15% of the total hydrogen produced must be recycled into the system. However, this

compressor decouples the system from the fuel cell, thus providing the opportunity to store

hydrogen in a pressurized tank. A huge number of small fuel cell applications is forecasted to come,

and they will all need their own hydrogen sources at the customer. Unfortunately, a price has to be

payed for all these advantages of the system, namely a relatively large amount of palladium.

Palladium is needed both in the membrane and in the catalyst for ring opening. The question as to

whether the system is cost-efficient compared to other fuel cell systems can only be answered by a

detailed economic analysis. In Table 5, a comparison is made between the novel system and a unit

based on partial oxidation.

Table 5: Comparison between partial oxidation and the new concept.

Composition Concentrations Enthalpy

Component [mol] [Vol.%] [kJ/mol]n-Decane 1.00 0.63 92.4Toluene 0.58 0.37 -92.4

Total 1.58 1.0 0.0



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A NOVEL ROUTE FOR CONVERTING AROMATICS INTO HYDROGEN VIA STEAM REFORMING.

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