Ammonia Process

Applicatio
n
Guide
Summary
Ammonia Process
This Application Guide
provides information
regarding upgrade and
retrofit opportunities in
the process of producing
ammonia.
Business Value
After becoming familiar
INTRODUCTION
The purpose of this document is to provide information regarding turbomachinery
upgrade and retrofit opportunities in the process of producing ammonia. First, we’ll
discuss ammonia as a product, how it’s produced, and what support systems this
requires. Then we’ll describe three areas of opportunity for Triconex: safety, turbine
controls and compressor controls. We’ll go into detail regarding what elements
we can install, upgrade, or retrofit within those three categories. To conclude, we’ll
discuss implications and strategies, special considerations, and other important
information. You’ll find a number of diagrams following the text that describe
relevant processes and organization.
with the process involved
AMMONIA AS A PRODUCT
in ammonia production,
Ammonia is primarily used in the production of fertilizers, such as ammonium
sulfate, ammonium nitrate, ammonium phosphate. It is also used as a refrigerant
and as an intermediate for some petrochemicals. Most ammonia plants are part
of a larger fertilizer manufacturing facility that work with nitric acid, phosphoric
acid, sulfuric acid, urea and/or co-gen. In its pure form, ammonia exists at ambient
conditions as a gas. When compressed to between 200 and 300 psig, it can be
condensed against air or cooling water. In order to apply it to the soil, ammonia
must be combined with other substances. These substances are chosen for their
additional benefits to the soil.
it’s easy to see how crucial
the related turbomachinery
equipment is. It’s also easy
to see the importance
of reliable controls in
minimizing safety hazards
and increasing efficiency.
With extensive, field-proven
experience, Invensys
Triconex provides control
systems guaranteed to
improve ammonia plant
performance.
PRODUCING AMMONIA
The basic feedstocks for ammonia are natural gas (methane) and air. Natural gas
provides hydrogen, and air provides nitrogen to the NH3 ammonia molecule. A
conventional ammonia plant has these basic components: a primary reformer, a
secondary reformer, a carbon dioxide removal and a synthesis loop. Most ammonia
plants also have support and utility sections, steam system integration, storage
facilities, etc.
In the primary reformer, high-pressure methane feed gas is heated and combined
with high-pressure steam. In the presence of the heat and a catalyst, the methane
and steam react to form carbon dioxide and hydrogen.
In the secondary reformer, the hot effluent of the primary reformer is combined with air, causing the
oxygen in the air to combine with hydrogen. This process results in hot carbon dioxide, nitrogen,
hydrogen and water vapor. These gases are cooled by generating steam.
Next, the process gas is passed through the carbon dioxide removal section and the dryers to
remove all but the hydrogen and nitrogen gases (please see Figure 1 on page 6 for greater detail).
This mixture is called the synthesis gas.
The synthesis gas is compressed by the syn gas compressor to more than 2500 psig. Then it is
circulated through the synthesis loop where a catalyst induces the nitrogen and hydrogen to react to
form ammonia. A direct contact cooler scrubs out the ammonia that forms in the syn loop. Unreacted
gases are returned to a recycle stage in the syn gas compressor, and to the syn loop. Liquid ammonia
from the ammonia refrigeration compressor condenser is sprayed into the reacted syn gas stream to
cool it as it passes through the contact cooler.
Because this process poses significant hazards to both turbomachinery and safety, Invensys Triconex
offers products and services to improve performance and increase user safety.
Developments in Ammonia Production
Prior to the late 1960s, first generation ammonia plants used reciprocating compressors because the
required pressures were very high (over 5000 psig) and the flows were low. A large plant produced
200 tons per day.
Through the development of low-pressure catalysts and larger furnaces in the mid-1960s, M. W.
Kellogg achieved pressures and flows that allowed the use of centrifugal compressors. Other
ammonia developers soon began using similar designs. These second generation plants were
capable of producing 1000–1250 tons per day.
A third generation plant is now being built by a few designers, including Kellogg and KTI/Fish. These
plants are characterized by even lower syn loop pressures and modular process designs.
SUPPORT SYSTEMS
A number of support systems are associated with the production of ammonia. Some of these systems
exist in almost every ammonia plant, while others are provided according to specific needs. This
section discusses three specific support systems: feed gas compression, feed gas treating, and
steam systems.
•• Feed Gas Compression. When the natural gas supply to the plant is lower than the process requires,
a compressor boosts the feed to the pressure required by the primary reformer. This compressor is
usually a small, single-section centrifugal model driven by a steam turbine or an electric motor.
•• Feed Gas Treating. If the feed gas contains sulfur or mercaptans, a desulfurization unit eliminates
these contaminants prior to the gas’s entry in the primary reformer.
•• Steam Systems. As the secondary reformer burns the oxygen (with hydrogen), a tremendous
amount of heat becomes available and is used to generate steam. While older ammonia plants produced 600 psig steam, secondary reformers in post-1970s plants produced 1500–1800 psig steam. This high-pressure steam production became practical with the development of high-speed steam turbines suitable for such pressures.
To start the ammonia process, an auxiliary boiler provides the 600 psig steam required in the primary
reformer. This steam drives the process air compressor and the ammonia refrigeration compressor.
Ammonia plants usually pass 1500 psig steam from the secondary reformer through the
high-pressure section of the syn gas compressor turbine. This is sometimes called “topping the
turbine.” The steam extracted from the high-pressure section is used to supply the process and the
turbines operating at 600 psig. The condensing section of the syn gas compressor turbine provides
any extra horsepower required by the compressor that is not already produced in the high-pressure
section of the turbine.
Other steam pressure levels—such as 250, 150 and 50 psig—are used for heating and for small steam
turbines in the plant. The syn gas compressor turbine inlet valve (V1) controls the 1500 psig steam
pressure. The auxiliary boiler, let down and vent valves control the 600 psig header pressure.
WHAT’S IN IT FOR TRICONEX
When the Soviet Union dissolved, most of their inefficient ammonia plants were decommissioned
because of excessive auto-consumption costs. Since that time, the profit margin in western-style
plants has improved—especially because modern ammonia plants eliminate equipment redundancy.
They now depend on sophisticated, un-spared furnaces and compressors, which deliver high profit
margins. This environment is the perfect setting for the Invensys Triconex controllers.
Ammonia is an important component of the HPI market. To produce ammonia, manufacturing and
fertilizer companies require robust control systems for their unique turbomachinery and safety needs.
Invensys’ goal is to support these systems with our portfolio of solutions and services.
•• Safety. Although ammonia plants now generate a high profit margin, they also face significant
safety issues. In fact, their end product (NH3) is lethal if aspirated.
One major safety concern relates to high pressures and temperatures. For example, the primary
reformer is a fired heater that raises the methane temperature to over 1000°F. The secondary
reformer adds high-pressure air to the effluent of the primary reformer. Both of these pieces of
equipment represent very significant explosion risks if they are not properly protected. Another
safety concern involves the synthesis loop, which contains hydrogen at up to 3000 psig. This highpressure hydrogen can cause extensive damage if it is not precisely controlled.
All of these safety hazards require the use of a very robust Emergency Shutdown System (ESD)
platform. Invensys’ TMR safety systems are designed to protect equipment and people, minimizing
risks by implementing proper safety routines and reliable hardware.
•• Turbine Controls. Because steam is required in the production process, and because the secondary
reformer has a significant exotherm, ammonia plants generally utilize steam turbines for driving
pumps and compressors. In some ammonia plants (C. F. Braun design), gas turbines are used for
driving the syn gas and/or process air compressors. The current Braun design still uses a Frame 5
to drive the air compressor. In this case, the exhaust is ducted into the primary reformer for use as
heated combustion air.
These turbines require reliable and efficient turbomachinery controls. Invensys Triconex has
significant experience in the retrofit, upgrade and commissioning of TMC equipment involved in
ammonia production. We provide turbine services for the following elements:
Process Air Compressor. This compressor is usually driven by a single-valve condensing turbine
with 600 psig inlet steam. As mentioned above, this compressor is sometimes driven by a gas
turbine or an electric motor.
Syn Gas Compressor. These compressors routinely operate at over 12,000 rpm. Their most
common driver is a steam turbine, as this eliminates the need for a gear box. Gas turbines in this
compressor’s horsepower range operate at 5000 rpm or less. In the mid-1906s, M. W. Kellogg
pioneered the use of a very high pressure extraction-condensing steam turbine for this service.
Today, virtually all designs incorporate this arrangement.
Ammonia Refrigeration Compressor. A 600 psig inlet single-valve extraction turbine is the most
common driver for this service.
Feed Gas Compressor. Sometimes called the Natural Gas Compressor, this machine boosts feed
gas to the pressure required by the primary reformer. Usually, it is a small barrel compressor
driven by a single-valve steam turbine or an electric motor. Some ammonia plants do not have a
fuel gas compressor if their pipeline pressure is adequate.
Combustion Air Fans. Large fans are used for the primary reformer combustion air. The induced
draft (ID) fan is usually three times the size and power of the forced draft (FD) fan.
Boiler Feed Pumps. To protect the steam system against an electrical power failure, at least one
of the boiler feed water pumps is driven by a steam turbine.
CO2 Removal Circulating Pumps. These pumps are usually large—similar in power to the boiler
feed, but spared by a motor-driven pump. Considering the large condenser loads in an ammonia
plant, cooling water flows are significant. There is usually at least one cooling water pump, which
is driven by a condensing steam turbine.
•• Compressor Controls. Each of the process compressors mentioned above is unspared and
critical to the process. If any one of these machines stops producing forward flow, the process is
interrupted. Each of the following machines also has special control requirements:
Feed Gas Compressor. The compressor is operated on discharge pressure control. Whether this
machine is driven by motor or turbine, it usually includes a suction throttle valve that handles
any uncertainty in suction pressure. For example, if the load drops such that the turbine reaches
minimum governor and the discharge pressure continues to rise, the suction throttle valve
dissipates this additional pressure energy.
Using an integrated controller cures a number of problems. For instance, the discharge pressure
control is split-ranged between the speed controller and the suction throttle valve controller.
The integrated controller is also useful in operating the compressor bypass valve (which differs
from the kickback valve) for start-up and shutdown of the train.
Process Air Compressor. The 600 psig discharge from this machine enters the secondary
reformer. To generate a pressure ratio higher than 40 requires up to six intercooled stages of
compression. Plants built prior to the 1990s used two compressor bodies, and the high-pressure
body sometimes used a gearbox.
Most modern ammonia plants use a single, integrally-geared six-stage compressor. The twobody compressors use a modulated vent valve on the discharge to prevent surge during
operation above the minimum governor speed. Below minimum governor speed, a solenoidoperated vent valve opens on the low-pressure body discharge to protect the low-pressure body.
Syn Gas Compressor. In a second generation plant, this two-body machine has three intercooled
stages and a recycle stage. It is common for the turbine to have a compressor body on each end.
The second generation Braun plants use a three-body machine (three stages plus recycle)
driven by a gas turbine—usually a Pratt and Whitney or Rolls-Royce aeroderivative. Third
generation plants have reduced the syn loop pressure such that the syn gas compressor fits into
a single body.
CONCLUSION
Now that we have discussed the process involved in ammonia production, it’s easy to see how crucial
the related turbomachinery equipment is. It’s also easy to see the importance of reliable controls in
minimizing safety hazards and increasing efficiency. With extensive, field-proven experience, Invensys
Triconex provides control systems guaranteed to improve ammonia plant performance.
DIAGRAMS
The following diagrams explain the process of ammonia production for Kellogg, Linde, and KTI/Fish
Processes:
Ammonia Process (Figure 1)
Fuel Gas Compressor (Figure 2)
Air Compressor—Modern Kellogg Process, Third Generation (Figure 3)
Syn Gas and Ammonia Train—Modern Kellogg Process, Third Generation (Figure 4)
Process Air Compressor—Modern Linde Process (Figure 5)
Nitrogen Booster Compressor—Modern Linde Process (Figure 6)
Syn Gas Compressor—Modern Linde Process (Figure 7)
Turbo Generator Set—Modern Linde Process (Figure 8)
Syn Gas/Ammonia Train—Modern KTI/Fish Process (Figure 9)
Process Air Compressor—Modern KTI/Fish Process (Figure 10)
Process Air Compressor—Kellogg Traditional Process (Figure 11)
Ammonia Refrigeration Compressor Train—Kellogg Traditional Process (Figure 12)
Synthesis Gas Compressor Train—Kellogg Traditional Process (Figure 13)
CO2 + 4H2
Methane (CH4)
CH4 +
2H2O ==
CO2 + 4H2
CO2 + 4H2
+ N2 + O2
== CO2 +
N2 + 2H2O
+ 2H2
Steam
CO2 + N2 +
2H2O + 2H2
Air
Fuel Gas
CO2 Removal
Secondary
Reformer
Primary
Reformer
Synthesis Gas Compressor
Dryers
Ammonia Refrigeration Compressor
Synthesis Loop
Figure 1: Ammonia Process
To Process
PT
Motor Starter
UIC
Compressor
Kickback
Valve
Motor
PT
FT
PC
From Pipeline
Figure 2: Fuel Gas Compressor
Inlet
Filter
PT
FT
2
3
TT
1
1
2
TT
Motor Starter
Motor
4
4
TT
PT
TT
3
PT
5
6
5
PT
Figure 3: Air Compressor—Modern Kellogg Process, Third Generation
Syn Gas
T&T Valve
V1
PT
1800 psig
V2
UIC
PRV
PT
1
PT
FT
FT
2
Turbine
NE
PT
TT
600 psig
2
NE
PT
Ammonia
Refigeration
1
3
4
NE
FT
PT
NRV
TT
FT
FT
PT
UIC
600 psig
PT
UIC
HIC
PC
PT
PT
From Process
IP
LP
HIC
UIC
HP
Figure 4: Syn Gas and Ammonia Train—Modern Kellogg Process, Third Generation
Inlet
Filter
FT
2
TT
1
1
2
Motor Starter
Motor
3
5
FT
TT
Figure 5: Process Air Compressor—Modern Linde Process
From Air Separation
Process
FT
2
TT
1
1
2
Motor
3
5
TT
PT
Figure 6: Nitrogen Booster Compressor—Modern Linde Process
V1
PT
T&T Valve
110 bar
PRV
V2
PT
UIC
PT
3
FT
UIC
TT
R
Turbine
NE
30 bar
NE
1
2
NE
PT
NRV
TT
PT
PT
FT
PT
TT
Process Recycle
PT
30 bar
From Process
Syn Gas
Modern Linde Process
Figure 7: Syn
GasCompressor,
Compressor—Modern
Linde Process
T&T Valve
V1
30 barg
V2
PRV
MWT
Turbine
Generator
NE
NE NE
PT
5-6 barg
Figure 8: Turbo Generator Set—Modern Linde Process
600# steam
PT
PT
Syn Gas
2
NE
FT
PT
NE
1
NH3
NE
FT
FT
FT
PT
PT
PT
From
Process
From
Process
Figure 9: Syn Gas/Ammonia Train—Modern KTI/Fish Process
1
PT
2
FT
3
6
7
TT
1
2
TT
TT
4
8
4
PT
TT
3
PT
5
5
6
9
PT
Figure 10: Process Air Compressor—Modern KTI/Fish Process
Air Compressor IIC Overview
FIC
FIC
03
04
S
1
A
101-L
HCV
048
148-F
PT
LT
D
13A
LCV
101-JC2
FT
B
TT
PT
FE03
LIC
0101
I
TT
S
FCV
023
2
TT
P
FE04
FT
0101
PT
2MCL605
2MCL357
Air to CO2 compressor
Gear
PT
SE
Plant & Instrument air
101-JC1
PIC
SIC
101-JC3
147-F
LT
149-F
LIC
LT
LIC
0102
0100
I
I
P
P
LCV
2 Valve Algorithm
0102
F
E
LCV
0100
Figure 11: Process Air Compressor—Kellogg Traditional Process
M
AMMONIA COMPRESSOR ITCC OVERVIEW
To NH3
Vent
To
127-C
PT
2MCL 456
3MCL 456
SE
PT
PT
PT
PT
PIC
SIC
PT
TT
167-C
TT
128-C
PIC
FE
012
2 Valve Algorithm
PT
FT
PT
FE
011
FIC
012
FT
FE
010
FT
FT
09
FIC
010
FIC
09
TT
TT
120 CF 1
120 CF2
LT
120 CF3
LT
I
I
P
P
P
P
LIC
LIC
120 CF4
I
LT
I
LT
FE
FIC
011
LIC
LIC
FV
012
FV
011
FV
010
FV
09
I
I
I
I
P
P
P
P
Liquid
NH3
From
127-C
LV
LV
From Accumulator
LV
LV
Figure 12: Ammonia Refrigeration Compressor Train—Kellogg Traditional Process
SYN GAS COMPRESSOR ITC OVERVIEW
I
P
To
109-DA/B
I
A
P
From
105-LA/LB
FV7
From
114-C
I
116-C
PT
115-C
PIC
6
LV6
TT
PIC
7
FT
FIC
8
FE7
B
120-C
FV59
FIC
59
TT
PT
LT
104-F
FT
PT
BCL
458
LIC
5
PT
LT
I
I
SE
P
P
PT
TT
PT
LIC
11
134-C
PT
FE8
103-L
FT
FE59
105-F
2BCL 458/A
LV11
PIC
P
C
SIC
LV5
2 Valve Algorithm
Figure 13: Synthesis Gas Compressor Train—Kellogg Traditional Process
121-C
Invensys • 10900 Equity Drive, Houston, TX 77041 • Tel: (713) 329-1600 • Fax: (713) 329-1700 • iom.invensys.com
Invensys, the Invensys logo, ArchestrA, Avantis, Eurotherm, Foxboro, IMServ, InFusion, SimSci, Skelta, Triconex, and Wonderware are trademarks of Invensys plc, its subsidiaries or affiliates.
All other brands and product names may be the trademarks or service marks of their representative owners.
© 2013 Invensys Systems, Inc. All rights reserved. No part of the material protected by this copyright may be reproduced or utilized in any form or by any means, electronic or mechanical, including
photocopying, recording, broadcasting, or by any information storage and retrieval system, without permission in writing from Invensys Systems, Inc.
Rel. 10/13
PN TR-0133