Talk about the valve application of Ammonia Synthesis

Introduction

Ammonia Synthesis is the key link of nitrogen fertilizer production, and nitrogen fertilizer is an indispensable raw material for agricultural production. Ammonia is also an important raw material in the chemical industry for the production of amines, anilines, amides and nitric acids. The by-products of ammonia synthesis, such as carbon monoxide and sulfur, can be used as feedstocks for other chemical processes. This highlights the important role of synthetic ammonia in the national economy. In this paper, the selection of valve in ammonia plant is discussed with reference to "gas raw material to Braun process".

Overview of Valve Applications

In the utility systems, auxiliary production systems, and other systems with moderate conditions, the valve selection is straightforward. This article will focus on the specialized valves used in the ammonia synthesis production plant.

1. Desulfurization Section

Process Characteristics:

• Similar to mild hydrodesulfurization units.
• Media: Hydrogen, hydrogen sulfide, hydrocarbon gases.
• Pressure: ~4.2 MPa
• Temperature: 380-420°C

Valve Types:

• Shut-off Valves: Gate valves and globe valves are commonly used. Ball valves are less preferred due to cost and the use of centrifugal compressors for raw gas compression.
• Butterfly Valves: Not advantageous due to the small pipeline diameter.

Valve Structure:

• Use low-leakage structures or materials for hydrogen-containing pipelines.
• Typical pressure allows for bolted bonnet valves, with flange or welded end connections.

Valve Materials:

• For high-temperature hydrogen and hydrogen sulfide pipelines, material selection is guided by Nelson and Copper curves.
• <260°C: Carbon steel
• ≥260°C: 1.25Cr-0.5Mo
• >350°C: Corrosion-resistant materials, possibly stabilized austenitic stainless steel (321).
• Materials must have high purity to handle hydrogen.
• For wet hydrogen sulfide environments, valves should meet NACEMR0103/MR0175 standards for SSC resistance.

2. Conversion Section

Process Characteristics:

• High temperatures, up to 800-1000°C in catalyst tubes, with media temperatures up to 500°C within process piping.
• Pressure: ~3.2 MPa
• Produces high-pressure steam (10.5 MPa, 315°C).

Valve Types:

• Gate valves and globe valves are used for shut-off.
• Butterfly valves are not suitable due to high temperature.

Valve Structure:

• Low-leakage materials/structures for hydrogen and CO2 pipelines.
• Bolted bonnet valves with flange or welded connections.
• High-pressure steam valves with pressure-seal bonnets, welded connections, and bypass for DN200 or larger.

Valve Materials:

• For hydrogen pipelines (>400°C), use 2.25Cr-1Mo materials based on Nelson curves.
• High-pressure steam pipelines can use carbon steel due to graphitization constraints at 315°C.

3. Shift Conversion Section

Process Characteristics:

• Media temperatures up to 450°C
• Pressure: ~2.0 MPa
• Media includes hydrogen, nitrogen, CO2, CO (before shift), and methane (after shift).

Valve Types:

• Gate valves and globe valves are used for shut-off.

Valve Structure:

• Low-leakage materials/structures for hydrogen and CO.
• Valve materials should handle hydrogen corrosion; use 2.25Cr-1Mo for >400°C.

4. Gas Purification Section

Process Characteristics:

• Similar to acidic water gas units in refineries.
• Media includes hydrogen and CO2, with a risk of severe carbon steel corrosion.

Valve Types:

• Full-bore valves recommended; ball valves for shut-off.
• Low-leakage packing structures for valves.

Valve Materials:

• Carbon steel for ≤260°C, 1.25Cr-0.5Mo for ≥260°C.
• Prevent stress corrosion cracking in amine-rich environments.

5. Compression and Ammonia Synthesis Section

Process Characteristics:

• High temperature, high pressure, and hydrogen presence.
• No hydrogen sulfide but includes ammonia.

Valve Types:

• Full-bore ball valves to reduce pressure drop.

Valve Structure:

• Low-leakage structures, pressure-seal bonnets, welded connections.

Valve Materials:

• For temperatures >350°C with ammonia, use SS321 to avoid chromium metal damage from nitrogen atoms.

6. Refrigeration Section

Process Characteristics:

• Presence of liquid ammonia.
• Low-temperature media conditions.

Valve Types:

• Gate, globe, ball, and butterfly valves, depending on specific requirements.
• 17-4PH materials for valve stems, Babbitt alloy for sealing faces.
• Use ball valves with non-metallic seals for very low temperatures.

Conclusion

Valve selection in ammonia synthesis plants must consider the specific process characteristics, including media composition, temperature, and pressure. Appropriate valve types, structures, and materials ensure safety, efficiency, and longevity in the demanding conditions of ammonia production.

FAQs

1. What are the key considerations for valve selection in high-temperature hydrogen environments?
   • Valve materials should have high purity and be resistant to hydrogen corrosion, often requiring materials like 2.25Cr-1Mo for temperatures above 400°C.
2. Why are ball valves less preferred in some sections of the ammonia synthesis plant?
   • Ball valves are more expensive, and their advantages are not significant in systems using centrifugal compressors and small-diameter pipelines.
3. What measures are taken to prevent stress corrosion cracking in the gas purification section?
   • Use full-bore valves, low-leakage packing structures, and post-processing stress relief treatments on pressure components.
4. How are valves chosen for the refrigeration section involving liquid ammonia?
   • Materials like 17-4PH for valve stems and Babbitt alloys for sealing faces are used to handle low temperatures and prevent ammonia stress corrosion cracking.
5. What factors influence the selection of valve materials in the shift conversion section?
   • Hydrogen presence and media temperatures guide material choices, with 2.25Cr-1Mo preferred for high-temperature, hydrogen-containing pipelines.