Ammonia Synthesis Catalysts: Innovation And Practice

Innovation and Practice

Huazhang Liu

• Publisher: WSPC
• Year: 2013
• Print ISBN: 9789814355773
• eBook ISBN: 9789814518802
• Topic: Biological Sciences
• Subtopic: Science General
• Index: Biological Sciences

This book provides a review of worldwide developments in ammonia synthesis catalysts over the last 30 years. It focuses on the new generation of Fe 1- x O based catalysts and ruthenium catalysts — both are major breakthroughs for fused iron catalysts. The basic theory for ammonia synthesis is systematically explained, covering topics such as the chemical components, crystal structure, preparation, reduction, performance evaluation, characterization of the catalysts, the mechanism and kinetics of ammonia synthesis reaction. Both theory and practice are combined in this presentation, with emphasis on the research methods, application and exploitation of catalysts.

The comprehensive volume includes an assessment of the economic and engineering aspects of ammonia plants based on the performance of catalysts. Recent developments in photo-catalysis, electro-catalysis, biocatalysis and new uses of ammonia are also introduced in this book.

The author, Professor Huazhang Liu, has been engaged in research and practice for more than 50 years in this field and was the inventor of the first Fe 1-x O based catalysts in the world. He has done a lot of research on Fe 3 O 4 based- and ruthenium based-catalysts, and has published more than 300 papers and obtained 21 patents during his career.

Contents:

• Historical Evolution of Catalysts for Ammonia Synthesis
• Catalytic Reaction Mechanisms of Ammonia Synthesis
• Chemical Composition and Structure of Fused Iron Catalysts
• Preparation of Fused Iron Catalysts
• Reduction of Fused Iron Catalysts
• Ruthenium Based Ammonia Synthesis Catalysts
• Performance Evaluation and Characterization of Catalysts
• Performance and Application of Catalysts
• Effect of Catalyst Performance on the Economic Benefits of Catalytic Process
• Innovation and Speculation

Readership: Researchers in academia and industry working on catalysts for ammonia synthesis.

Chapter 1

Historical Evolution of Catalysts for Ammonia Synthesis

Catalytic ammonia synthesis technology has played a central role in the development of the chemical industry during the 20th century. This industrial importance has been paralleled by a significant scientific interest in understanding and improving the ammonia synthesis catalyst. Often new techniques, methods, and theories of catalysis have initially been developed and applied in connection with studies of this system. Similarly, new discoveries in the field of ammonia synthesis have been extended to other areas of catalysis. The combined influence of refined characterization techniques, improved kinetic analysis, and new possibilities in theoretical modeling, has led to a detailed insight into the fundamentals of ammonia synthesis catalysts. Several recent reviews give a comprehensive account of the current understanding.

Ammonia is primarily used as nitrogenous fertilizer and as a raw material of inorganic compounds including nitric acid, ammonium salts, cyanide and organic compounds, such as amines, sulfanilamide and so on. In addition, ammonia is also an excellent refrigerant. Since ammonia is a key raw material for industry and agriculture, the process of ammonia synthesis has an extremely important position in any economy.

In the 19th century, ammonia was obtained from natural saltpeter or recovered from coal. In order to meet the increasing demand for nitrogenous fertilizers, a variety of methods were tried to fix nitrogen from air at the beginning of the 20th century. From 1902 to 1913, three nitrogen-fixing processes were created, i.e., the electric arc process, calcium cyanamide process and catalytic ammonia synthesis technology.¹

The electric arc method (1902) which produces nitric oxide via reaction of nitrogen with oxygen at the high temperatures under the electric arc was inspired by the fulmination phenomena in nature. Then, nitric oxide is further oxidized by oxygen in air into nitrogen dioxide, followed by adsorbtion in water to form nitric acid. About 50–80 kW · h of electric energy is required to convert one kg of nitrogen. High energy consumption limited the wide application of this process in industry.

The cyanamide process is based on the formation of calcium cyanamide (CaC₂ + N₂ = Ca(CN)₂ ) through the reaction of calcium carbide (CaC₂ ) with nitrogen. Calcium carbide is produced by the reaction of calcium oxide and carbon at high temperatures. Calcium cyanamide can be either directly used as nitrogenous fertilizers or as a raw material to produce cyanides and nitrogen containing organic compounds. To fix one kg of nitrogen by this cyanamide process, the electrical energy consumption was about 16–18 kW · h, which is only a quarter of that consumed by the arc process. The cyanamide process was widely applied in Europe and was the major method for nitrogen fixation before the First World War.

Catalytic ammonia synthesis from N₂ and H₂ was developed by Fritz Haber, and then Carl Bosch applied this process to industrial production successfully at the beginning of 20th century. The first ammonia plant was built for the production of 30 ton of ammonia per day in 1913 at Oppau, Germany. Up to 1934, this catalytic process became the dominant route for nitrogen fixation. Because of the extensive use of nitrogenous fertilizers, catalytic ammonia synthesis plays an important role in agriculture and other industries. The huge market demand drives larger production scale and complexity of the equipments and processes of production. Nowadays, because ammonia synthesis requires extensive capital investment and massive energy consumption and the price of products is low, the development of industrial ammonia synthesis process emergences in following features² :

(1) Increasing in scale of unit equipment. With the increasing demand for ammonia, the scale of unit equipment is continuously increased to decrease the investments and production costs. At present, the production capacity of single-stream equipment has reached 1,850 ton of ammonia per day.

(2) Unceasing innovations in its process including catalysts. Different types of catalysts are used in ammonia plant. The improvement of catalysts not only increases the efficiency of production, but also saves consumption of the power. For example, the pressures of ammonia synthesis were 20–30 MPa and even 100 MPa in the early years. Now the pressure has been decreased to 8–10 MPa over the new developed catalysts. Because these catalysts are sensitive to impurities in synthesis gas, innovation in purification technology of synthesis gas arises and develops.

(3) Wide application of various energy-saving technologies. Because the major raw material of synthesis ammonia is fuel, the price of ammonia largely depends on the price of energy resource, and the energy-saving technologies are becoming more and more important with the increase of the price of energy resource. To decrease the unit energy consumption, the pressure of syngas production was increased and the high-pressure steam produced by the reaction heat is recovered as power. With the development of ammonia synthesis technology, the energy consumption has been decreased to 27 GJ in advanced modern ammonia plants, which is very close to the theoretical value of 20 GJ.

1.1 Introduction of Catalysts for Ammonia Synthesis

Although a lot of technology progress has been achieved, the basic principles and process in modern ammonia plants are essentially the same as original ones developed by Haber and Bosch, a century ago. The ma jor procedures can be outlined as follows²:

(1) Production of synthesis gas. Synthesis gas, containing hydrogen and carbon monoxide, is produced by the reaction of steam with solid fuels such as coals, cokes, heavy oil, light hydrocarbon or gaseous hydrocarbon such as natural gas at high temperatures. During this procedure, certain amount of air, oxygenenriched air or oxygen are introduced, wherein oxygen is combusted with fuels to provide heat for reactions and produce carbon monoxide. Following reactions, the residue nitrogen remains in the gas for ammonia synthesis. When pure oxygen is used, nitrogen has to be supplied from air separation unit. Carbon monoxide formed during these reactions is converted to hydrogen and carbon dioxide by the water-gas shift reaction.

(2) Purification of synthesis gas. The sulfur- and carbon-containing compounds in synthesis gas must be removed in order to avoid the poisoning of catalysts in the following processes. Sulfur and carbon containing compounds are absorbed by different solvents. The used solvents are regenerated by desorption and H₂ S (or element S) and carbon dioxide are recovered. The trace amounts of carbon monoxide and carbon dioxide which remained in synthesis gas is removed via the reaction of methanation or other methods. After a series of purifications, the content of carbon monoxide and carbon dioxide in the synthesis gas are on the levels of ppm (1 ppm = 1 ml · m^–3 ).

(3) Compression of synthesis gas. The synthesis gas composed by hydrogen and nitrogen is compressed to required pressures, usually 10–30 MPa, by pistontype or centrifugal compressors. During ammonia synthesis, single-pass conversion is only 10%–20%, and therefore, most of the synthesis gas must be recycled, compressed and returned to the synthesis loop again.

(4) Synthesis and separation of ammonia. After exchanging heat with the hot gas which comes from the reactor, synthesis gas is introduced into the reactor to form ammonia over the catalyst. Following the reaction, the gas containing ammonia reaches heat exchanger to generate steam under high-pressures, and then is cooled down by water and ammonia. Ammonia in syngas is condensed into liquid and separated from the syngas. The remaining syngas is recycled to the synthesis loop again. Figure 1.1 shows the typical schematic diagram for ammonia synthesis using different feedstock.

During ammonia synthesis, the major reactions of production and purification of synthesis gas and the synthesis of ammonia, all are carried out over different catalysts. At least eight kinds of catalysts are used in the whole process, where natural gas or naphtha is used as feedstock and steam reforming is used to produce synthesis gas. These catalysts are Co–Mo hydrogenation c...

Table of contents

• Cover
• Ammonia Synthesis Catalysts
• Title
• Copyrights
• AbouttheAuthor
• Preface
• Foreword
• Author
• Contents
• Chapter 1
• Chapter 2
• Chapter 3
• Chapter 4
• Chapter 5
• Chapter 6
• Chapter 7
• Chapter 8
• Chapter 9
• Chapter 10
• Index