Introduction To Ammonia Production

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BACK TO BASICS
SEPTEMBER 2016
VENKAT PATTABATHULA, JIM RICHARDSON
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Ammonia is critical in the manufacturing of fertilizers, and is one of the
largest-volume synthetic chemicals produced in the world. This article
explores the evolution of ammonia production and describes the

current manufacturing technologies.
Most people associate the pungent smell of ammonia (NH3) with cleaners or
smelling salts. However, the use of ammonia in these two products represents only a
small fraction of the total global ammonia production, which was around 176
million metric tons in 2014 (1). To appreciate where the industry and technology are
today, let’s first take a look at how we got here.
Ammonia has been known for more than 200 years. Joseph Priestley, an English
chemist, first isolated gaseous ammonia in 1774. Its composition was ascertained by
French chemist Claude Louis Berthollet in 1785. In 1898, Adolph Frank and
Nikodem Caro found that N2 could be fixed by calcium carbide to form calcium
cyanamide, which could then be hydrolyzed with water to form ammonia (2):
CaO + 3C ↔ CaC2 + CO
CaC2 + N2 ↔ CaCN2 + C
CaCN2 + 3H2O ↔ CaCO3 + 2NH3
The production of significant quantities of ammonia using the cyanamide process
did not occur until the early 20th century. Because this process required large
amounts of energy, scientists focused their efforts on reducing energy requirements.

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German chemist Fritz Haber performed some of the most important work in the
development of the modern ammonia industry. Working with a student at the Univ.
of Karlsruhe, he synthesized ammonia in the laboratory from N2 and H2.
Meanwhile, Walther Nernst, a professor of physical chemistry at the Univ. of Berlin,
developed a process to make ammonia by passing a mixture of N2 and H2 across an
iron catalyst at 1,000°C and 75 barg pressure. He was able to produce larger
quantities of ammonia at this pressure than earlier experiments by Haber and

others at atmospheric pressure. However, Nernst concluded that the process was
not feasible because it was difficult or almost impossible (at that time) to produce
large equipment capable of operating at that pressure.
Nonetheless, both Haber and Nernst pursued the high-pressure route to produce
ammonia over a catalyst. Haber finally developed a process for producing
commercial quantities of ammonia, and in 1906 he was able to achieve a 6%
ammonia concentration in a reactor loaded with an osmium catalyst. This is
generally recognized as the turning point in the development of a practical process

for the production of ammonia in commercial quantities.
Haber realized that the amount of ammonia formed in a single pass through a
converter was far too low to be of commercial interest. To produce more ammonia
from the makeup gas, he proposed a recycle system, and received a patent for the
concept. Haber’s recycle idea changed the perception of process engineering as
static in favor of a more dynamic approach. In addition to the chemical reaction
equilibrium, Haber recognized that reaction rate was a determining factor. Instead
of simple yield in a once-through process, he concentrated on space-time yield in a
system with recycle.
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^ purchased Haber’s patents and started development of
BASF a commercial
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After testing more than 2,500 different catalysts, Carl Bosch, Alvin Mittasch, and
other BASF chemists developed a promoted iron catalyst for the production of
ammonia in 1910. Developing equipment that could withstand the necessary high
temperatures and pressure was an even more difficult task. An early mild steel
reactor lasted only 80 hours before failure due to decarbonization. Lining mild steel
reactors with soft iron (which was not vulnerable to decarbonization) and adding
grooves between the two liners to release hydrogen that had diffused through the
soft iron liner solved this problem. Other major challenges included designing a
heat exchanger to bring the inlet gas to reaction temperatures and cool the exit gas,
and devising a method to bring the catalyst to reaction temperature.
The first commercial ammonia plant based on the Haber-Bosch process was built by
BASF at Oppau, Germany. The plant went on-stream on Sept. 9, 1913, with a
production capacity of 30 m.t./day.
▲Figure 1. This is a simplified flowsheet of the first commercial ammonia plant by BASF.
Figure 1 is a flowsheet of the first commercial ammonia plant. The reactor contained
an internal heat exchanger in addition to those shown on the schematic.
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Global production rates
▲Figure 2. Worldwide ammonia production has steadily increased from 1946 to 2014.
Ammonia production has become one of the most important industries in the world.
Without the crop yield made possible by ammonia-based fertilizers and chemicals,
the global population would be at least two to three billion less than it is today (3).
Ammonia production has increased steadily since 1946 (Figure 2), and it is
estimated that the annual production of ammonia is worth more than $100 billion,
with some plants producing more than 3,000 m.t./day of NH3.
In 1983, on the occasion of the 75th anniversary of AIChE’s founding, a blue ribbon
panel of distinguished chemical engineers named what they believed to be the
world’s ten greatest chemical engineering achievements (4). Embracing such feats
as wonder drugs, synthetic fibers, and atomic energy, the citation also included the
breakthrough that permitted the production of large quantities of ammonia in
compact, single-unit plants.

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Withinexperience
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that make vastyou

websites, amounts
consent toof ammonia
our at relatively low costs. As recently as 80 years
use of cookies.
ago, the total annual production of synthesized ammonia was just over 300,000
m.t. Thanks to chemical engineering breakthroughs, one modern ammonia plant

can produce more than 750,000 m.t./yr.
Approximately 88% of ammonia made annually is consumed in the manufacturing
of fertilizer. Most of the remainder goes into the production of formaldehyde. China
produced about 32.6% of the global production in 2014, while Russia, India, and the
U.S. produced 8.1%, 7.6%, and 6.4%, respectively (1). While most of the global
production of ammonia is based on steam reforming of natural gas, significant
quantities are produced by coal gasification; most of the gasification plants are
located in China.
Modern production processes
The tremendous increase in ammonia demand from 1950 to 1980 necessitated

larger, more-energy-efficient plants. Those decades also saw a change in design
philosophy. Until that time, an ammonia plant was regarded as an assembly of

unrelated units, such as gas preparation, gas purification, gas compression, and
ammonia synthesis. New innovations and an integral design tied process units
together in the most effective and efficient ways.

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▲Figure 3. KBR designed one of the first single-train, large-capacity ammonia plants.
In the mid-1960s, the American Oil Co. installed a single-converter ammonia plant
engineered by M.W. Kellogg (MWK) at Texas City, TX, with a capacity of 544
m.t./day. The single-train design concept (Figure 3) was so revolutionary that it
received the Kirkpatrick Chemical Engineering Achievement Award in 1967.
The plant used a four-case centrifugal compressor to compress the syngas to a
pressure of 152 bar, and final compression to an operating pressure of 324 bar
occurred in a reciprocating compressor. Centrifugal compressors for the synthesis
loop and refrigeration services were also implemented, which provided significant
cost savings.
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using a centrifugal compressor as part of the synthesis gas compression
maximizing the recovery of waste heat from the process
generating steam from the waste heat for use in steam turbine drivers
using the refrigeration compressor for rundown and atmospheric refrigeration.
An integrated scheme that balanced energy consumption, energy production,
equipment size, and catalyst volumes was incorporated throughout the plant.
Most plants built between 1963 and 1993 had large single-train designs with
synthesis gas production at 25–35 bar and ammonia synthesis at 150–200 bar.
Another variation by Braun (now KBR) offered slight modifications to the basic
design. The Braun Purifier process plants utilized a primary or tubular reformer
with a low outlet temperature and high methane leakage to reduce the size and cost
of the reformer. Excess air was added to the secondary reformer to reduce the
methane content of the primary reformer exit stream to 1–2%. Excess nitrogen and
other impurities were removed downstream of the methanator. Because the

synthesis gas was essentially free of impurities, two axial-flow ammonia converters
were used to achieve a high ammonia conversion.
Some recently built plants have a synthesis gas generation system with only one
reformer (no secondary reformer), a pressure-swing adsorption (PSA) system for H2
recovery, and an air separation plant as the source of N2. Improvements in

converter design, such as radial and horizontal catalyst beds, internal heat
exchangers, and synthesis gas treatment, helped increase ammonia concentrations
exitingOur
the synthesis converter from about 12% to 19–21%. A higher conversion per
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pass, along with more-efficient turbines and compressors, further reduced energy
consumption. More-efficient CO removal solutions, such as potassium carbonate

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and methyldiethanolamine (MDEA), have contributed to improved energy
efficiency. Most modern plants can produce ammonia with an energy consumption
of 28 GJ/m.t.
In addition to the design, mechanical, and metallurgical improvements made during

this time, the operating pressure of the synthesis loop was significantly reduced.
When the first single-train plant was built in the 1960s, it contained a high-pressure
synthesis loop. In 1962, MWK received an inquiry from Imperial Chemical
Industries (ICI) for a proposal to build a 544-m.t./day plant at their Severnside site.
MWK proposed a 152-bar synthesis loop instead of a 324-bar loop.
Because the development of kinetic data for the ammonia reaction at 152 bar would
take more time than MWK had to respond to the ICI inquiry, they contacted Haldor
Topsøe to support their plans. Topsøe had data covering the entire pressure range of
interest to MWK. In addition, they had a computer program for calculating the
quantity of catalyst that was required at the lower operating pressure. Even though
ICI chose Bechtel to design the plant, MWK was able to develop a flowsheet for a
544-m.t./day design with centrifugal compressors and a low-pressure synthesis

loop, which some people consider the single most important event in the
development of the single-train ammonia plant.
Approximately twice as much catalyst was required at 152 bar as at 324 bar, an
increase that seemed economically feasible. Although the converter would need
twice the volume, the lower operating pressure would reduce the required thickness
of the pressure shell. As a result, the weight of metal required for the converter plus
the catalyst remained about the same. The lower-pressure synthesis loop also
allowed the use of centrifugal compressors instead of reciprocating
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Another improvement was recovering heat to generate high-pressure steam for
steam turbine drives.
Plant designs in the 21st century
During the first few years of the 21st century, many improvements were made in
ammonia plant technology that allow existing plants to increase production rates
and new plants to be built with larger and larger capacities. Competition between
technology suppliers is quite fierce. Three technology licensors — KBR (Kellogg

Brown and Root), Haldor Topsøe, and ThyssenKrupp Industrial Solutions (TKIS) —
currently dominate the market. Ammonia Casale, which offers an axial-radial

catalyst bed design, is a market leader in revamps of existing plants.
▲Figure 4. Modern ammonia plants designed by KBR employ its proprietary Purifier design.
Most of the ammonia plants recently designed by KBR utilize its Purifier process
^ Our
(Figure websites use cookies to offer you a better browsing
4), which combines low-severity reforming in the primary
Got it reformer,
More infoa liquid
N2 wash purifier downstream of the methanator to remove impurities and adjust
the H2:N2 ratio, a proprietary waste-heat boiler design, a unitized chiller, and a
horizontal ammonia synthesis converter.
Depending on the configuration of the plant, energy consumption can be as low as
28 GJ/m.t. Because the secondary reformer uses excess air, the primary reformer
can be smaller than in conventional designs. The cryogenic purifier (shown in
Figure 4 in light green with a light orange background), which consists of an
expander, condenser, feed/effluent exchanger, and rectifier column, removes

impurities such as CO, CH4, and argon from the synthesis gas while adjusting the
H2:N2 ratio of the makeup gas in the ammonia loop to the optimum level. The
ammonia concentration exiting the low-pressure-drop horizontal converter is 20–
21%, which reduces energy requirements for the recycle compressor. KBR also
offers a low-pressure ammonia loop that employs a combination of magnetite
catalyst and its proprietary ruthenium catalyst.
▲Figure 5. Haldor Topsøe offers an ammonia plant design that has a proprietary side-fired
^ reformer
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offer you supply
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The syngas generation section (or front end) of a Haldor Topsøe-designed plant
(Figure 5) is quite traditional with the exception of its proprietary side-fired
reformer, which uses radiant burners to supply heat for the reforming reaction.
Haldor Topsøe also offers a proprietary iron-based synthesis catalyst, radial-flow
converters consisting of one, two, or three beds, and a proprietary bayonet-tube
waste-heat boiler. More recent developments include the S-300 and S-350
converter designs. The S-300 converter is a three-bed radial-flow configuration with
internal heat exchangers, while the S-350 design combines an S-300 converter with
an S-50 single-bed design with waste-heat recovery between converters to maximize
ammonia conversion.
▲Figure 6. ThyssenKrupp’s dual-pressure synthesis loop design features a once-through

reactor between syngas compressors.
ThyssenKrupp offers a conventional plant (Figure 6) with a unique secondary

reformer design, a proprietary waste-heat boiler, radial-flowGot
converters,
it Moreand
info a dual-
pressure ammonia synthesis loop. Today, a production rate of 3,300 m.t./day can
be achieved using the TKIS dual-pressure process.
▲Figure 7. The Linde Ammonia Concept (LAC) features a pressure-swing adsorption unit for
high-purity hydrogen production and an air separation unit for high-purity nitrogen production.
The Linde Ammonia Concept (LAC) is an established technology process scheme

with over 25 years of operating experience in plants with capacities from 200
m.t./day to over 1,750 m.t./day. The LAC process scheme (Figure 7) replaces the
costly and complex front end of a conventional ammonia plant with two well-
proven, reliable process units:
production of ultra-high-purity hydrogen from a steam-methane reformer with

PSA purification
production of ultra-high-purity nitrogen by a cryogenic nitrogen generation

unit, also known as an air separation unit (ASU).

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▲Figure 8. Ammonia Casale’s process employs a catalyst bed that harnesses axial-radial
technology, which has a lower pressure drop and higher efficiency than standard catalyst beds.
Ammonia Casale’s plant design has a production rate of 2,000 m.t./day. One of the
key features of this design is axial-radial technology in the catalyst bed (Figure 8).
In an axial-radial catalyst bed, most of the synthesis gas passes through the catalyst
bed in a radial direction, creating a very low pressure drop. The rest of the gas
passes down through a top layer of catalyst in an axial direction, eliminating the
need for a top cover on the catalyst bed. Casale’s axial-radial catalyst bed technology
is used in both high-temperature and low-temperature shift converters, as well as in

the synthesis converter.
Other technologies
Some technology suppliers have offered gas-heated reformers (GHRs) for the
production of ammonia in small-capacity plants or for capacity increases. Unlike
conventionally designed plants that use a primary reformer and secondary reformer
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operating in series, plants with GHRs use the hot process gas from the secondary
reformer to supply heat to the primary reformer. This reduces the size of the
primary reformer and eliminates CO2 emissions from the primary reformer stack,
making the process more environmentally friendly.
Even though some ammonia producers advocate for distributed production of

ammonia in small ammonia plants, most companies prefer to build large facilities
near cheap raw material sources and transport the product by ship, rail, or pipeline
to the consumers.
Ammonia from coal
▲Figure 9. China produces most of its ammonia from coal.
China produces more ammonia than any other country, and produces the majority
of its ammonia from coal (Figure 9).
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The basic processing units in a coal-based ammonia plant are the ASU for the
separation of O2 and N2 from air, the gasifier, the sour gas shift (SGS) unit, the acid
gas removal unit (AGRU), and the ammonia synthesis unit. Oxygen from the ASU is
fed to the gasifier to convert coal into synthesis gas (H2, CO, CO2) and CH4. There
are many gasifier designs, but most modern gasifiers are based on fluidized beds
that operate above atmospheric pressure and have the ability to utilize different coal
feeds. Depending on the design, CO levels of 30–60% by volume may be produced.
After gasification, any particulate matter in the synthesis gas is removed and steam
is added to the SGS unit. The SGS process typically utilizes a cobalt and
molybdenum (CoMo) catalyst specially designed for operation in a sulfur

environment.
After reducing the CO concentration in the synthesis gas to less than 1 vol%, the
syngas is fed to an AGRU, where a chilled methanol scrubbing solution (e.g.,
Rectisol) removes CO2 and sulfur from the synthesis gas. The CO2 overhead is either
vented or fed to a urea plant. The sulfur outlet stream is fed to a sulfur recover unit
(SRU).
Syngas that passes through the AGRU is typically purified by one of two methods:
a nitrogen wash unit to remove residual CO and CH4 from the syngas before it
is fed to the synthesis loop
a PSA system for CO and CH4 removal.
Closing thoughts
During
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Plant layouts evolved
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and analyze sitemulti-train designs,
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preparation in the front end of the plant increased from atmospheric pressure to
30–50 barg pressure. Capacities increased from 100 m.t./day to as much as 3,300
m.t./day in a single train.
Energy efficiencies have improved as well — from consumptions well above 60
GJ/m.t. of ammonia in coke-based plants to 40–50 GJ/m.t. in the first natural-gas-

based plants to 30–40 GJ/m.t. in the first single-train plants. Modern plants have
added heat recovery by steam production at pressures as high as 125 barg in both
the syngas preparation section and the synthesis loop.
In terms of process equipment, there has been a shift from reciprocating

compressors to centrifugal compressors. An internal heat exchanger has been
implemented in the synthesis converter to increase conversion of H2 and N2 to NH3.
Designers have tapped into hydrogen recovery from purge gas (in units such as PSA
systems) to enhance production or reduce the plant energy consumption. Designers

have also implemented hot feed gas desulfurization systems. There have been
significant improvements in the catalysts used in reforming, shift conversion,
methanation, and ammonia synthesis.
To improve process control and safety, distributed control systems (DCSs) for
advanced process control, as well as safety-instrumented systems (SISs), are now
standard in ammonia plants. Before any process goes online, hazard and operability
(HAZOP) studies and layer of protection analyses (LOPAs) are performed. Advances
in training simulators and education practices ensure that operators and engineers
can perform their duties safely and effectively.
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^ are just a few of the thousands of improvements in technology
These Got it and info
More safety that
have been implemented to make the ammonia industry one of the most productive
and safe industries in the world.
Literature Cited
1. U.S. Geological Survey, “Nitrogen (Fixed) — Ammonia Statistics,”
minerals.usgs.gov/minerals/pubs/historical-statistics/ds140-nitro.xlsx (Last
Modified: Jan. 28, 2016).
2. Slack, A. V., and G. R. James (eds.), “Ammonia,” Parts I, II, and III,
Marcel Dekker, New York, NY (1974).
3. Smil, V., “Enriching the Earth – Fritz Haber, Carl Bosch, and the
Transformation of World Food Production,” The MIT Press, Cambridge, MA
(Dec. 2000).
4. Williams, G., and V. Pattabathula, “One Hundred Years of Ammonia

Production — A Recap of Significant Contributions to Feeding the World,” 58th
Annual Safety in Ammonia Plants and Related Facilities Symposium, AIChE
(Aug. 25–29, 2013).
Acknowledgments
The authors acknowledge the assistance of KBR, ThyssenKrupp Industrial

Solutions, Haldor Topsøe, Linde, and Casale for providing technical literature on
their respective process technologies.
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Home Publications CEP September 2016 Introduction to Ammonia Production

66th Annual Safety in Ammonia Plants and Related Facilities

VENKAT PATTABATHULA, JIM RICHARDSON

^ Our websites use cookies to offer you a better browsing

explores the evolution of ammonia production and describes the

CaCN2 + 3H2O ↔ CaCO3 + 2NH3

The production of significant quantities of ammonia using the cyanamide process

amounts of energy, scientists focused their efforts on reducing energy requirements.

Meanwhile, Walther Nernst, a professor of physical chemistry at the Univ. of Berlin,

developed a process to make ammonia by passing a mixture of N2 and H2 across an

quantities of ammonia at this pressure than earlier experiments by Haber and

large equipment capable of operating at that pressure.

commercial quantities of ammonia, and in 1906 he was able to achieve a 6%

ammonia concentration in a reactor loaded with an osmium catalyst. This is

generally recognized as the turning point in the development of a practical process

concept. Haber’s recycle idea changed the perception of process engineering as

static in favor of a more dynamic approach. In addition to the chemical reaction

of simple yield in a once-through process, he concentrated on space-time yield in a

system with recycle.

Our websites use cookies to offer you a better browsing

production capacity of 30 m.t./day.

websites, you consent to our use of cookies.

with some plants producing more than 3,000 m.t./day of NH3.

panel of distinguished chemical engineers named what they believed to be the

breakthrough that permitted the production of large quantities of ammonia in

compact, single-unit plants.

that make vastyou

m.t. Thanks to chemical engineering breakthroughs, one modern ammonia plant

Approximately 88% of ammonia made annually is consumed in the manufacturing

Modern production processes

The tremendous increase in ammonia demand from 1950 to 1980 necessitated

philosophy. Until that time, an ammonia plant was regarded as an assembly of

together in the most effective and efficient ways.

^ Our websites use cookies to offer you a better browsing

websites, you consent to our use of cookies.

received the Kirkpatrick Chemical Engineering Achievement Award in 1967.

The plant used a four-case centrifugal compressor to compress the syngas to a

websites, you consent to our use of cookies.

maximizing the recovery of waste heat from the process

using the refrigeration compressor for rundown and atmospheric refrigeration.

An integrated scheme that balanced energy consumption, energy production,

other impurities were removed downstream of the methanator. Because the

were used to achieve a high ammonia conversion.

recovery, and an air separation plant as the source of N2. Improvements in

consumption. More-efficient CO removal solutions, such as potassium carbonate

In addition to the design, mechanical, and metallurgical improvements made during

544-m.t./day design with centrifugal compressors and a low-pressure synthesis

technology suppliers is quite fierce. Three technology licensors — KBR (Kellogg

currently dominate the market. Ammonia Casale, which offers an axial-radial

Depending on the configuration of the plant, energy consumption can be as low as

expander, condenser, feed/effluent exchanger, and rectifier column, removes

websites, you consent to our use of cookies.

▲Figure 6. ThyssenKrupp’s dual-pressure synthesis loop design features a once-through

ThyssenKrupp offers a conventional plant (Figure 6) with a unique secondary

The Linde Ammonia Concept (LAC) is an established technology process scheme

production of ultra-high-purity hydrogen from a steam-methane reformer with