Lloyd L. Handbook of Industrial Catalysts

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388 Chapter 9

completed, more slowly, at the highest inlet temperature possible up to a maxi-

mum of 450°C.

9.6.3. Other Methanation Processes

Methanation catalysts or, alternatively, pressure-swing absorption are used to

purify synthesis gas and refinery hydrogen throughout the world. The methana-

tion reaction has also been applied in other interesting and useful processes alt-

hough not yet on a very significant scale.

When natural gas was felt to be running short in the US the British Gas

Council CRG (Catalytic Rich Gas) process was converted to produce a substi-

tute natural gas process simply by adding two methanation reactors to the origi-

nal town gas process. The rich gas, including steam, was cooled and then

methanated at 300°C limiting the outlet temperature to less than 400°C. The gas

was cooled again and the remaining oxides of carbon were converted to methane

in a second reactor, with a rise in temperature of only about 40

C, as shown in

Table 9.16. Unalkalized CRG catalysts were used in the methanation reactors.

Other processes were introduced by Lurgi and Japan Gasoline but, in the end,

there no need for additional supplies of natural gas and the plants were only op-

erated for a short time. It was also shown that the product from coal gasification,

which contained oxides of carbon, was an alternative feed for conversion to me-

thane.

Pure hydrogen can be recovered by a methanation procedure from olefin

plant tail gas that contains methane, ethylene and carbon monoxide. Typical

methanation catalysts will remove up to 0.3% carbon monoxide and 0.3% eth-

ylene at 270°C, 30 atm pressure and about 6000 h

-1

space velocity. A catalyst

which does not crack ethylene to form carbon but which can produce ethane or

TABLE 9.16. Catalytic Rich Gas (CRG) Reforming and Substitute Natural Gas (SNG).

Process Composition (dry outlet gas)

Naphtha Feed CRG SNG

Two-stage methanation/CO

removal

20–22 <1

CO 1 –

16–18 <1

64 98.5

Temperature inlet (°C)

450 300 300

Temperature outlet (°C)

510 375 340

Pressure (atm) >10 >10 >10

Steam ratio weight 2 steam / 1 naphtha

Hydrogen recycle 0.06 m

/ kg naphtha

0.01 m

/ kg propane

Synthesis Gas

389

methane by hydrogenation, should be chosen for the duty. Carbon monoxide is

completely removed by this procedure.

The EVA-ADAM process was proposed as a means whereby energy could

be transferred over fairly long distance. The process was based on a combination

of the steam reforming and methanation reactions. The energy required to fuel

the endothermic steam reforming reaction could be obtained from any remote

source, such as a nuclear reactor, in contrast to the normal process in which gas

is burnt to supply the heat. The products from the steam reforming reaction con-

tained both hydrogen and the oxides of carbon. Energy, in the form of super-

heated steam, was then recovered from the exothermic heat of methanation at a

second location. Methane was also available as fuel if necessary. The high tem-

perature of the operation in the methanators, at 700°C, 500°C and 300°C, inlet

the three reactors, required that special active, yet heat resistant, catalysts were

necessary. The catalysts were deactivated by a combination of sulfur poisoning

and carbon formation, through partial regeneration was possible by treatment

with hydrogen at 500°C. The reaction product contained about 80% methane.

Adiabatic beds could be replaced by internally cooled, tubular reactors (IRMA).

The maximum temperature in tubes reached more than 600°C. The process has

an exit temperature of 320°C giving up to 83% methane. Once again, the pro-

cess has not yet been commercialized.

9.7 OTHER APPLICATIONS OF STEAM REFORMING

The steam reforming process with typical catalysts has been widely used to pro-

duce a range of hydrogen rich gases. The examples in Table 9.17 show how the

operating conditions have been modified to obtain the appropriate gas composi-

tion.

9.7.1 Methanol Synthesis Gas

The gas produced by steam reforming any hydrocarbon using normal catalysts

can be used directly as methanol synthesis gas with no further treatment. When

methane is used as feedstock for the steam reformer, the gas produced contains

50% more hydrogen than that which is required for the methanol synthesis reac-

tion, and this gas must be recovered at some stage in the process. However, as

the molecular weight of the hydrocarbon increases, the amount of excess hydro-

gen produced falls with increasing molecular weight. When naphtha is used as

the feedstock, the resulting synthesis gas is virtually stoichiometric for methanol

synthesis. This is illustrated in the following equations:

390 Chapter 9

TABLE 9.17. Other Applications of Steam Reforming Process.

Process Lean town gas OXO alcohol

synthesis gas

Direct iron ore

reducing gas

Feed Naphtha Natural Gas Methane & Top Gas

Steam/carbon 3 <2 2

Inlet pressure (atm) 27 15 5

Temperature inlet (°C)

~450 540 400–450

Temperature outlet (°C)

750 870 950

Outlet gas composition (vol%):

Hydrogen 60 64 74

Carbon monoxide 10 22 24

Carbon dioxide 16 11 1

Methane 12 3 1-2

+ H

O Æ CO + 3 H

(9.15)

2n+2

+ n H

O Æ n CO + (2n + 2) H

(9.16)

n CO + 2n H

Æ n CH

OH (9.17)

Synthesis gas from natural gas feeds has, however, an excess of hydrogen but

may be used directly if a high purge rate from the synthesis loop is acceptable.

It is possible to adjust the ratio of hydrogen to carbon oxides by the addition

of carbon dioxide to the synthesis gas, either just before the methanol synthesis

loop, or directly into the feed for the steam reformer. It is also possible to use a

secondary reformer using oxygen rather than air, after the primary reformer. The

amount of oxygen used is adjusted to provide the stoichiometric ratio of hydro-

gen and carbon oxides and, at the same time, reduce the amount of inert methane

in synthesis gas. (Table 9.9)

9.7.2 OXO Synthesis Gas

Typical steam reformers are used to produce synthesis gas for OXO reactions. It

is usual to operate with a steam/carbon ratio of less than two with a recycle of

sufficient carbon dioxide to provide a carbon dioxide/methane ratio of about two

so that an appropriate composition of the gas product is obtained.

9.7.3 Hydrogen Production

For the production of hydrogen, the steam reformer is operated at high severity

to obtain maximum methane conversion and early hydrogen plants used exactly

the same catalysts as ammonia plants, except for the secondary reformer. The

Synthesis Gas

391

hydrogen product is purified by catalytic conversion of the carbon monoxide,

removal of carbon dioxide followed by methanation. This product is then ac-

ceptable for use in most refinery and chemical processes (see Table 9.9).

More recently pressure swing absorption units using molecular sieves have

replaced the carbon dioxide removal and methanation steps and, often, the LTS

stage, to give very pure hydrogen. Only the HTS catalyst is retained to remove

most of the carbon monoxide. Hydrogen of even greater purity can be obtained

by operating at a low steam ratio in the steam reformer.

9.7.4 Reducing Gas

The direct reduction of iron ore in blast furnaces decreases the amount of high

quality coke required and can also lead to an increase in the production capacity

of the furnace. Gas with a high content of hydrogen and carbon monoxide is

produced at low pressure when natural gas is reformed using an almost stoichi-

ometric steam to carbon ratio. However, the low steam ratio results in a tenden-

cy for the deposition of carbon. Some of the top gas from the blast furnace,

which has a carbon dioxide content of about 20%, is recycled to the steam re-

former to compensate for the low steam-to-carbon ratio and to provide the nec-

essary volumes of hydrogen and carbon monoxide. The gas flow is upwards

through the tubes, which are filled with large-diameter rings of catalyst to re-

duce pressure drop. The feed gas is pre-heated over inert rings, or even low ac-

tivity catalyst, before passing into the reformer stage in which high-activity,

thermally-stable catalysts are used (see Table 9.17).

9.7.5 Town Gas Production

Town gas was first made on a large scale by the Chartered Gas Company in

London during 1812. Production then spread throughout Europe and the USA.

The discovery of natural gas in California during 1928 and then in other states

led to its domestic use in the US particularly after long-distance pipelines were

installed. By 1934 almost 40% of the US domestic supply came from natural gas

and this increased rapidly to more than 95% by 1960. During the 1970s, when it

was thought that US natural gas supplies were declining, about ten substitute

natural gas (SNG) plants were built in the eastern states using the UK Gas

Council’s catalytic rich gas (CRG) process. (Table 9.16)

The burners used in domestic appliances at that time were not suitable for

use with natural gas, because of its high calorific value. For this reason, natural

gas was not used directly until the 1960s. Small cyclic reforming processes were

introduced to meet peak demand when the supply of coal gas was insufficient to

meet the requirements. Larger plants were installed to meet the needs of larger

areas, and the relatively low calorific value of the reformer gas was increased to

the required level by the addition of low-molecular-weight hydrocarbons.

392 Chapter 9

A wide range of catalysts produced by impregnating α-alumina or magnesia

with low concentrations of nickel oxide were used in cyclic reformers. These

catalysts had to be extremely heat resistant because the process generated carbon

which was removed at intervals by burning in air.

By 1960 the ICI naphtha steam reforming process was being used to make

ammonia synthesis gas from the very cheap naphtha then available from Euro-

pean refineries. This process was easily adapted to make town gas simply by

operating at lower reforming temperature. The reformed gas then contained a

higher methane concentration. The calorific value of the gas was increased from

385Btu per cubic foot to 500Btu per cubic foot by enrichment with hydrocar-

bons.

More than 200 of these lean gas plants were built throughout the world

before natural gas was used directly. (Table 9.17)

At about the same time the British Gas Council developed its adiabatic pro-

cess in which naphtha or lighter hydrocarbons were converted into a high me-

thane content gas with a calorific value of almost 700 Btu per cubic foot. This

gas could then be converted into town gas with a calorific value of 500 Btu per

cubic foot by continuous steam reforming in a lean gas plant and appropriate

blending. Since then the CRG process has also been widely used throughout the

world to produce methane rich gas from naphtha and LPG.

The CRG process operates adiabatically in a small single bed reactor. The

catalyst has high nickel oxide content and is precipitated under carefully con-

trolled conditions with alumina to give a high activity and thermally stable struc-

ture. The finished catalyst is alkalized to prevent carbon formation and is re-

duced before operation. Steam to carbon ratio can be as low as 1.5 when reform-

ing naphtha to produce town gas.

When operating with naphtha as feed, the endothermic reforming reaction is

dominant at the top of the bed. As the concentrations of both hydrogen and car-

bon monoxide increase, the exothermic methanation reaction becomes the dom-

inant reaction so that the overall reaction is exothermic. Gradual catalyst deacti-

vation leads to the temperature profile moving down the bed and catalyst life

can be as long as two years. With natural gas feed any hydrocarbons heavier

than methane are reformed, the methane steam equilibrium is established, and

the overall reaction is endothermic. Operating details are shown in Table 9.16.

9.7.6 Substitute Natural Gas

Substitute natural gas (SNG) may be obtained by methanation of CRG process

gas with a stable nickel-alumina catalyst after removal of the carbon dioxide

formed in the conventional process. Gas containing more than 98% methane can

be produced. The CRG process is operated at low steam ratio to maximize me-

thane production. Two stages of methanation follow the CRG reactor and inter-

cooling is required to control the temperature. Standard alkalized, high nickel

CRG

catalys

9.7.7

In an

laced

relativ

ondar

surize

catalys

conver

carbon

swing

succes

lyst-fil

1300º

simila

steam/

Figure 9

atalyst is use

t in the two m

utothermal

ttempt to im

in some plan

ly small ICI

reformer and

primary ref

ts are used.

carbon mon

dioxide, resi

adsorption sy

fully.

n another pr

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within a sing

to that of t

atio. The co

.9. Combined st

in the reform

thanation sta

Reformin

rove energy

designs by

mmonia plan

the hot proce

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single

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tem. Both pl

cess, tested b

combined wit

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e CRG proc

position of th

am reformer/a

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efficiency, th

n autothermal

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s gas used to

ed versions

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gen at about

noxide and i

nts were rapi

Uhde, refor

the partial o

re 9.10). The

ss and oper

product gas

Synt

tothermal refor

a non-alkaliz

steam refor

reforme

(Fi

excess air is

provide heat

f the conven

rature shift c

700–2800

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or a small, pr

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carbons in c

reformed ga

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Figure 9.10. Model of the Uhde ammon

ia plant. Reprinted

ith permission from Uhde GmbH.

394 Chapter 9

Synthesis Gas

395

achieved by variation in the steam and feed rates. The methane content was de-

pendent upon the amount of oxygen used in the enriched air, or the flow of oxy-

gen to the partial oxidation section of the process. The process could be adjusted

to produce synthesis gas for methanol or OXO reactions by adding part of the

hydrocarbon feed directly to the oxidation section. The appropriate carbon mon-

oxide/carbon dioxide and hydrogen/carbon monoxide ratios in the gas composi-

tion were obtained by adjusting the volume of secondary feed and the

steam/carbon ratio of the primary feed, respectively.

REFERENCES

1. Alwin Mittasch, Early Studies of Multicomponent Catalysts, Advances in

Catalysis, Vol. 2, Academic Press, N. Y., 1950, p. 81.

2. German patents 292615 (1913), 279582 (1913), and 293585 (1914); US Pa-

tent 1330772 (1918); British Patents 27955 (1912), 8864 (1913), and 27963

(1913).

3. Green, Industrial Catalysis, Ernest Benn, London, 1928, p. 327; Partington,

J. Soc. Chem. Ind. 40 (1921) 99R.

4. M. Appl, Brief History of ammonia production, Nitrogen, No. 100, British

Sulphur Publishing (March-April 1976), p. 49.

5. Byrne, Gohr, Haslam, Ind. Eng. Chem., 24(10) (1932) 1129.

6. Gard, Nitrogen, No 39 (Jan-Feb 1966) 25.

7. Chemical Week Report (Ammonia) (Sept 11 1965) 11.

8. Lewis B. Nelson, History of the US fertilizer industry, Tennessee Valley Au-

thority, 1990, p. 227.

9. British patents 953877 and 1003702 (1960s); Bridger, Safety in ammonia

and related facilities, AIChE Symposium, Boston (Sept 7-10 1975).

10. Catalyst Handbook, Ed. byM. V. Twigg, Wolfe Publishing Co, London,

1989; M. Appl, Modern ammonia technology, Nitrogen, No. 200, British

Sulphur Publishing, 1992.

11. Private communication, Alberta Nitrogen Products Ltd, Alberta, Canada.

12. Materials Technology in Steam Reforming Processes, Edeleanu, Pergamon

Press, London, 1966; J. K. Rostrup-Nielson, Catalysis, Science &

Technology, Vol. 5, Ed. By Anderson and Boudart, Springer Verlag, Berlin,

1984, p. 1; Howe-Baker, Hydrocarbon Processing (Nov 1995) 25 and (May

2000) 39.

13. Oil Gas Journal Special, (March 16 1998) 64; Cromarty and Crewdson, ICI

Symposium Thai Cat (1990).

14. Gas Making and Natural Gas, BP Trading Ltd, London, 1972.

AMMONIA AND METHANOL

SYNTHESIS

10.1 AMMONIA SYNTHESIS

In the early days, the composition of catalysts discovered and developed by Mit-

tasch that were used by BASF in the plants at Oppau and Leuna was kept secret.

Apart from the fact that iron was used and that the process operated at high pres-

sure, little other technical information was available until after the war had end-

ed. When the new synthesis process was investigated in other European coun-

tries and the United States, many different catalyst promoters were listed in a

large number of patent applications and in the technical literature.

The wide

variety of processes and catalysts developed during the 1920s reflects the diffi-

culties experienced in establishing suitable operating conditions. These technical

difficulties, together with a static demand for ammonia, meant that only a few

plants were actually built before 1940 and BASF retained a virtual monopoly on

the manufacture of ammonia.

Not all of the potential competitors used the catalyst promoters first intro-

duced by BASF and then later examined by Larson of the US Fixed Nitrogen

Research laboratory.

Other fused magnetite catalysts, containing promoters

such as magnesia, silica and chromium, as well as those with the more efficient

alumina, with potash and, perhaps calcium oxide, were used for a further twenty

to thirty years. These gave reasonable results in the converters then in use

shown in Table 10.1. Poor catalysts could operate adequately when the synthesis

gas was not purified efficiently and a short operating life was the norm.

L. Lloyd, Handbook of Industrial Catalysts, Fundamental and Applied Catalysis,

397