Lloyd L. Handbook of Industrial Catalysts
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348 Chapter 8
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Olefin Polymerization Catalysts 349
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350 Chapter 8
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9
SYNTHESIS GAS
Synthesis gas is a general term used in the chemical industry to describe a given
feedstock. Unfortunately the term means different gas compositions to different
groups of people and this can lead to confusion. In this chapter, the term synthe-
sis gas is used to describe the gaseous feed used specifically for the manufacture
of ammonia.
The replacement of charcoal by coke in the smelting of iron ores was a key
step in establishing coal as a primary raw material for the manufacture of chem-
icals. Coke was produced via the destructive distillation of coal. The by-
products included Town Gas, and a wide range of organic and inorganic com-
pounds which could be used in a variety of ways. Coal gas was used for domes-
tic illumination and, until natural gas became widely available, gas works were
important catalyst users mainly for gas purification. Coal tar contained organic
chemicals which were developed as intermediates in catalytic processes by the
emerging chemical industry. Ammonium sulfate was also recovered from the
process, and this became an important source of nitrogenous fertilizer. It was
also used in conjunction with phosphates that were extracted from phosphate
rock with sulfuric acid, to produce compound fertilisers. Prior to the recovery of
ammonia from coke ovens, it was only available from natural wastes. Conse-
quently, up to about 1920, most nitric acid was made from nitrates imported
from South America (Chile saltpetre), even though it was already known that
nitrates could be prepared via the catalytic oxidation of ammonia. By this time,
the coal-based water gas and producer gas processes for the manufacture of
town gas had been adapted to provide the synthesis gas needed for the Haber
process, and thus coal became the principal raw material for the manufacture of
ammonia.
L. Lloyd, Handbook of Industrial Catalysts, Fundamental and Applied Catalysis,
DOI 10.1007/978-0-387-49962-8_9, © Springer Science+Business Media, LLC 2011
351
352 Chapter 9
9.1 AMMONIA SYNTHESIS GAS
The importance of synthetic ammonia, when it was first produced by BASF in
1913, cannot be over emphasized.
1
Early development of the Haber process and
the widespread use of ammonia throughout the world will be described in Chap-
ter 10.
Since the successful commissioning of the first plant, the Haber-Bosch
process for the manufacture of ammonia has required increasingly large volumes
of synthesis gas as the demand, initially for explosives, and subsequently, ferti-
lizers continued to grow. For about 20 years, synthesis gas was made mainly
from coke in water gas and producer gas units, as shown empirically below:
C + H
2
O Æ CO + H
2
endothermic (9.1)
2 C + O
2
+ 4 N
2
Æ 2 CO + 4 N
2
exothermic (9.2)
The oxides of carbon are poisons to the iron-based ammonia synthesis cata-
lyst and must be removed from the synthesis gas before use. An important reac-
tion that affected the economics of the manufacture of synthesis gas was the
conversion of carbon monoxide into equal volumes of hydrogen and carbon di-
oxide by reaction with steam. This procedure, now commonly referred to as the
water-gas shift reaction required a catalyst based on iron and chromium oxides.
CO + H
2
O Æ CO
2
+ H
2
(9.3)
The carbon dioxide was washed out of the process gas with water at high
pressure and could eventually be recovered as a useful byproduct. Residual car-
bon monoxide was usually removed with an ammoniacal copper solution before
the final stage of ammonia synthesis.
The discovery of a shift catalyst by BASF was an important part of the pro-
cess development because it recovered the potential hydrogen from carbon
monoxide.
2
Originally known as the BAMAG method (Berlin Anhaltische Mas-
chinenbau Aktien Gesellschaft) the process has been used in every subsequent
major ammonia plant. There have been few changes in the catalyst since the first
application in 1914, other than more complete removal of impurities introduced
during catalyst manufacture, such as sodium and sulfate ions, and the use of
more stable formulations at increasingly high operating pressure. Operating
conditions for the first shift unit at Oppau are summarized in Table 9.1 and must
be amongst the first to be reported in the early literature.
3
Synthesis Gas
353
TABLE 9.1. The BAMAG Carbon Monoxide Conversion Process.
Feed vol% Inlet Catalyst Outlet Catalyst
CO
2
3–5 27–30
CO 35–40 2–4
H
2
33–36 50–52
N
2
22–23 16–18
Inerts <1 <1
Feed gas composed of 2–3 volumes of water gas combined with 1–2 volumes producer gas to give
required N
2
/H
3
ratio.
Gas rate (m
3
h
-1
) 70,000
Temperature (°C)
400–600
Square reactor (ft) 16x12(cross section)x10 deep
Catalyst beds (two) (ft) Each 16x12(cross section)x3 deep
Catalyst volume (m
3
) 30–35
Conversion (%) >90
Life (years) 2
Green, Industrial Catalysis, Benn, London, 1928, p. 327.
Partington, J. Soc. Chem. Ind. 46 (1921) 99R.
The strategic importance of ammonia, particularly in the production of ex-
plosives, led to competitive processes being developed in other parts of Europe
and North America. From the mid-1920s onward, it became clear that the
worldwide demand for fertilizers was becoming stagnant and further large-scale
expansion in capacity was not justified. This meant that process improvements
were delayed and even as late as 1937 the huge German wartime plants were
still providing about 70% of worldwide ammonia requirements.
4
Typical ammo-
nia plant capacity during this period was only 50–150 tonnes per day.
9.1.1 Process Developments
As a result of the post-war slump in the 1920’s, there was a momentum in both
Germany and the United Kingdom to be able to use surplus high-pressure
equipment in more effective ways. For example, the high-pressure catalytic pro-
cess for the hydrogenation of coal, introduced by Bergius in 1913, was eventual-
ly commercialized. However, the coal hydrogenation process provided not only
the anticipated liquid hydrocarbons to be used as gasoline but also large volumes
of byproduct off-gas.
Efforts were then made to improve the overall efficiency of the process
by using the off-gas as a feedstock for the manufacture of hydrogen, to reduce
the use of coke in the ammonia process. These efforts led to work on hydrocar-
bon steam reforming. A practical steam reforming process, using a solid nickel
catalyst in a simple tubular furnace, was soon developed by BASF. The process
was used for the first time by Standard Oil of New Jersey, during 1931, at their
Baton Rouge, Louisiana, and Bay Way, New Jersey,
5
refineries in early work on
the hydrocracking of gas oil to produce gasoline. During full scale operation of
354 Chapter 9
the steam reforming process, it was observed that the nickel catalysts suffered
from poisoning by sulfur compounds still present in the feed gas, and from the
deposition of carbon caused by olefin impurities. An improved furnace design,
containing tubes packed with a better nickel catalyst in the form of raschig rings,
was introduced by ICI in 1936 to make hydrogen from their own petrol plant
off-gas.
6
The catalyst was protected from poisons by desulfurizing the feed with
zinc oxide granules. Later on a cobalt molybdate catalyst was used, together
with the zinc oxide, to decompose the less reactive organic sulfur compounds.
By the 1940s it was recognized that methane could also be an excellent feed for
steam reformers in hydrogen or ammonia plants close to sources of natural gas.
Once again the onset of war led to an increased demand for ammonia and
several ordnance plants, using the ICI methane steam reforming process, were
built in North America.
7
These plants used the latest tubular reformer design,
with zinc oxide for feed desulfurization, and the raschig ring nickel catalyst sup-
ported on a cement based refractory. It is probable that the plants also included a
reforming vessel, with raschig ring nickel catalyst, to burn some of the hydrogen
in reformed gas with air to add nitrogen and produce synthesis gas.
8
Since a sec-
ondary reformer became part of the process, the original steam reforming fur-
nace has been called the primary reformer. The only other catalysts used in these
early plants were the original HTS catalyst, for the water gas shift reaction, and
an ammonia synthesis catalyst.
From 1950, the demand for nitrogen fertilizers in North America led to
the construction of many more ammonia plants all based on the steam reforming
process. Modifications to the primary reforming catalysts by the incorporation
of potash to reduce the level of carbon deposition have enabled operators in
those parts of the World with no readily available supply of natural gas to use
naphtha or refinery off-gases as feed for the primary reformer, and this has in-
creased the versatility of the process even further.
9
9.1.2 Increased Ammonia Production by Steam Reforming
Since 1945 the capacity of ammonia plants has grown to more than 2000 tonnes
per day and the operating pressure of the steam reformer has been increased
from atmospheric to about 30–40 atmospheres.
As the production capacity of ammonia plants grew from 50-100 tonnes
per day up to 2000 tonnes per day the process was considerably improved. A
more efficient furnace design was required which incorporated improved steel
alloys able to operate at higher pressure. At the increased operating pressure,
silica is volatile in steam. This required that the original catalysts, which were
made by precipitation and which were based on hydrous silica cements, needed
to be replaced by a new range of nickel-impregnated, silica-free catalysts. By
1966 large capacity, high-pressure single stream plants producing up to 1000
tons per day of ammonia were being built all over the world. These plants, how-
Synthesis Gas
355
ever, had evolved in stages as a range of new catalysts and equipment which
could operate a higher pressure became available.
Different versions of primary reforming and HTS catalysts, together with
several new catalysts able to withstand the severe operating conditions, were
gradually introduced. The rapid development of these new catalysts, designed
for complex plants, led to an increase in the number of chemical engineering
contracting companies able to supply off the peg process design and an extensive
catalyst technical service for operators. Commodity catalysts were soon available
from a number of specialist catalyst companies, which were not always in the in
the ammonia business themselves. These companies needed to carry out their
own catalyst research and plant design, so that such activities were no longer the
preserve of the large chemical companies, other than when processes were pro-
vided under a strict license agreement. Intense competition developed among the
new suppliers for the provision of new catalysts to independent operators, in
what became a large and profitable business sector. A consequence of the rapid
catalyst development, which was needed to solve operational problems, was that
most of the competing catalyst suppliers evolved somewhat different production
procedures and catalyst formulations.
The more severe process conditions that were being developed imposed
greater stresses, both physical and chemical, on the catalysts and the economic
penalty of premature catalyst failure required that catalyst reliability was of par-
amount importance in successful plant operation. Improvements in the testing of
catalysts was needed to provide a better understanding of the various modes of
catalyst failure, to meet the exacting standards now required by the users.
The major process changes and the new catalyst types which have been in-
troduced since 1920 are listed in Table 9.2. Relatively slow technical develop-
ments from 1920 up to about 1950 correspond to the period when coal was the
source of synthesis gas. Rapid changes between 1950 and 1970 reflect the ex-
pansion of the fertilizer and chemical industries as crude oil, natural gas, and
petrochemicals became so important. Increasing the level of fertilizer production
from natural gas was just as crucial as the new range of petrochemical products
supporting the tremendous increase in world population.
9.2 MODERN AMMONIA PLANTS
Up to eight different catalysts and absorbent materials are regularly used in a
modern ammonia plant to produce synthesis gas, and the design of any given
process is determined by the type of feedstock available. A wide range of differ-
ent catalyst compositions, shapes and sizes is now available to make operation
more reliable and economic. The selection of the best choice of catalysts for a
given process is made more complicated by the fact that several major suppliers
356 Chapter 9
Period Process Catalyst Developments
Pre 1930 Coal gasification. Desulfurization evolving (bog iron ore).
1930–40 Steam reforming developments
for hydrogen production.
Zinc oxide sulfur absorbent; raschig ring
reforming catalyst; secondary reforming
(in electrolytic hydrogen plants).
1940–55 Steam reforming process used in
ammonia plants. Natural gas
feed.
Sulfur absorption on activated carbon; sec-
ondary reforming; high temperature shift
catalysts used. Reformer-pressure increas-
ing from atmospheric to 9bar. Plant ca-
pacity increasing from 150 tpy to 300 tpy.
1960–63 Reforming process improvements
and alkalized catalysts intro-
duced. Use of naphtha feed.
Two-stage HTS conversion lowered CO
slippage to 1.0% and methanation includ-
ed. Plant capacity increased from 300 tpy
to 600/1000 tpy. LTS with methanation
replaced need for old CO scrubbing pro-
cesses. Operating pressure at least 30 atm.
offer different competitive products, each with its own set of advantages and
disadvantages. The complete range of catalyst types used in ammonia produc-
tion and typical operating lives are shown in Table 9.3. The overall process can
be divided into four parts and has been described previously:
10
• Feed purification. This can include hydrogenation catalysts to convert any
organic sulfur, nitrogen or chlorine to hydrogen sulfide, ammonia or hy-
drogen chloride, respectively, prior to removal by a suitable absorbent.
Early processes based on water gas/producer gas derived from coal had to
TABLE 9.3. Introduction of Ammonia Plant Catalysts and Typical Operating Life.
Catalyst Introduction Approximately operating life years
Desulfurization:
Activated carbon 1940 6
Zinc Oxide 1930 1–4 (depending on S content)
Cobalt molybdate 1930 5–10
Primary reforming:
Precipitated 1920s 2–10 (decreasing with severity)
Impregnated 1970s 2–5
Shapes 1980s 2–5
Alkalized 1959 2
Secondary reforming 1940+ 4–8
Carbon monoxide removal:
High temperature 1912 2–4
Low temperature 1963 2–4
Methanation 1960 5–10
Catalyst costs estimated at ~1.5% of total production costs.
TABLE 9.2. Ammonia Synthesis Gas Process Developments since 1920.
Synthesis Gas
357
deal with relatively large amounts of sulfur. The sulfur was removed in
very large oxide boxes which contained bog iron ore. Small, low-pressure
ammonia plants built before the introduction of the single-stream pro-
cesses often used activated carbon absorbents to remove the small
amounts of impurities present in natural gas. Modern plants use co-
balt/molybdenum hydrodesulfurisation catalysts to convert organo-sulfur
compounds to hydrogen sulfide prior to absorption by zinc oxide.
• Reforming. Hydrocarbons are converted catalytically to a mixture of hy-
drogen, nitrogen and oxides of carbon in two separate stages, namely
primary and secondary reforming.
• Oxygenate removal. Carbon monoxide is converted catalytically to hy-
drogen and carbon dioxide in two separate stages, the high- and low-
temperature shift reactions. Carbon dioxide is removed by any of the
available proprietary processes, and traces of residual carbon monoxide
are converted to methane over a nickel methanation catalyst. Any water
formed in this stage can be removed by molecular sieves, or by washing
with product ammonia.
• Ammonia Synthesis. This gives few problems in modern plants because
the synthesis gas is extremely pure. Although the iron synthesis catalyst
has not changed significantly since it was first used by BASF, a variety of
different converters, using pre-reduced catalyst, now make the process
more economic. A new, more active, catalyst made by impregnating an
active carbon support with ruthenium has been developed.
9.3 FEEDSTOCK PURIFICATION
Hydrocarbons that can be fed to ammonia plants include natural gas, associated
gas, liquid petroleum gas, and naphthas boiling up to 220°C. Higher hydrocar-
bons are not used in primary steam reforming because it would lead to coke
formation on the catalysts. Hydrocarbons are usually contaminated with variable
quantities of different sulfur compounds and often contain chlorides. These cata-
lyst poisons must be removed before the other catalysts in the plant can operate
in a satisfactory manner.
Impurities in natural gas are usually simple sulfur compounds such as hy-
drogen sulfide and mercaptans and are easily removed. Where the gas supply is
also used for domestic purposes a stenching agent such as a thioether or thio-
phene may have been added. These less-reactive compounds have to be subject-
ed to a hydrogenolysis treatment before they can be removed. Naphthas contain
several hundred parts per million of more complex organic sulfur compounds
and may be pre-treated in a hydrofiner before use in ammonia or hydrogen pro-
duction. Variation in the sulfur content of feed does not affect the performance