Best Valves for Enhanced PTA Processes

Introduction

Purified terephthalic acid (PTA) is the main raw material for polyester production. The global demand for polyesters for fibers, bottles, films, and other applications is driving the continued growth of PTA production. According to statistics, the world PTA production was about 6.6 million tons in 1990, about 24 million tons in 2000, about 34 million tons in 2005, and about 46.2 million tons in 2010. High temperature oxidation is a liquid phase air oxidation process using p-xylene as raw material. Reaction temperature 160-230℃. The process uses acetic acid as solvent, bromide as cocatalyst, and the oxidation product is crude terephthalic acid. Crude terephthalic acid was hydrogenated to produce purified terephthalic acid (PTA) as an intermediate for polyester production.

Valve Applications Overview

For the powder and gas flow transportation system in the plant, the inner walls of the entire piping system need special polishing treatment. The valves connecting to the equipment commonly include slide valves, rotary feeder valves, and discharge valves, with full-bore ball valves used in the piping system. Specific applications can be referenced from polyethylene plants. Other utility systems and auxiliary production systems involving medium and low-pressure media environments have similar valve applications to those described above. Below, we will focus on some unique valve applications in the PTA plant.

1. Crude Terephthalic Acid Production System

The crude terephthalic acid production system has two main characteristics:

1. The selection of primary valve materials in acetic acid environments.
2. The valves used in slurry systems.

In some cases, these two conditions coexist. Considering only the impact of acetic acid (including with and without bromine) on valve selection, acetic acid is a highly corrosive medium. Therefore, valves prone to liquid accumulation should not be used to avoid residual media that are hard to purge during shutdowns. Valves with low leakage structures should be used to prevent the escaping medium from damaging surrounding equipment. Valves should have good flow conditions since turbulence can accelerate corrosion.

There are numerous slurry media in the crude terephthalic acid production system, often coexisting with acetic acid. For example, valves used in pipelines before and after TA crystallization, filtration systems, and separation and drying systems all handle slurry media. The operating pressure for these systems is 0.8-1.3MPa, and the operating temperature is 190°C. For valves in slurry systems where acetic acid is also present, besides meeting anti-corrosion requirements, the following factors should be considered:

Key Considerations:

• Slurry Material Characteristics: The material is viscous and easily clogs. Thus, shut-off valves in these systems should have straight flow channels and be full-bore. Based on this analysis, plug valves and ball valves are preferred for slurry systems. Gate valves and globe valves are not suitable here due to high closing torque and resistance drop. Plug valves and ball valves offer good flow characteristics, excellent sealing, low operating torque, quick switching, minimal resistance, and easy maintenance, making them ideal for slurry, viscous fluids, and media requiring high sealing. Compared to plug valves, ball valves are relatively cheaper and more widely used.
• Ball Valves in PTA Plants: Various forms of ball valves are used, including general ball valves for shut-off and connection, V-ball valves for precise flow control, and three-way ball valves for medium distribution and flow direction change.
• Check Valves for High-Temperature, High-Pressure Environments: Flanged check valves are preferred because the bolts in wafer connection check valves have low stiffness and significant thermal expansion and contraction, reducing sealing reliability. In general, Y-type lift check valves are used for DN40 and below, while swing check valves are used for DN50 and above.
• Valve Structure: Generally, ball valves are required to be top-mounted for online maintenance and servicing.
• Low Leakage Valves: Valves with low leakage structures should be used when acetic acid is present to prevent damage to surrounding equipment.
• Other Requirements: These include selecting valve actuators, interlock requirements, bypass applications, material heat treatment, non-destructive testing, and transportation requirements, usually specified in project procurement technical specifications.

Additional Valves:

• Tank Bottom Slurry Discharge Valves: Used for discharging materials from the bottom of tanks, reactors, and other containers. These valves connect flanges directly to equipment pads at the bottom, eliminating the usual residue and clogging issues at the bottom outlet. Depending on the actual needs, discharge valves have two structural forms: rising and lowering type valve discs. The rising type is used for reactors with agitators (except frame and anchor type), while the lowering type is for reactors with frame and anchor type agitators.

General requirements for selecting tank bottom discharge valves include:

1. Straight-Through Structure: The valve type should be straight-through, adjustable, and not easily clogged. Thus, plunger-type valves are typically used.
2. Design to Prevent Clogging: The discharge valve body on the slurry pipeline generally welds a 200mm long section in the main pipeline flow direction. The plunger valve disc has a short stroke design, ensuring reliable sealing and effectively preventing material crystallization and reducing clogging.
3. Casting Inspection: Each discharge valve casting must undergo ultrasonic or radiographic inspection. The valve body interior is treated for abrasion and corrosion resistance, ensuring a certain surface hardness, providing high wear and corrosion resistance, and protecting the valve body from media erosion when the valve is opened.

2. Purified Terephthalic Acid Refining System

Compared to the oxidation reaction system, the acetic acid corrosion environment here is weaker, but the reaction pressure is higher. The system features multiple slurry and powder systems for transportation. For the slurry transportation system, refer to the oxidation reaction section for valve selection. For high-pressure systems, the valve selection requirements are as follows:

Key Considerations:

1. Low Resistance Drop: High-temperature, high-pressure media environments require valves with low resistance drop and reliable sealing. Using low resistance drop valves is a common requirement for high-pressure valves to minimize energy consumption by maintaining pressure with minimal losses.
2. Reliable Sealing: Sealing reliability is crucial for high-pressure valves. High temperatures and pressures exacerbate this issue, limiting some valve applications.
3. Concurrent High-Pressure and Slurry Conditions: Valves must meet both high-pressure and slurry media requirements when these conditions coexist.

Conclusion

In a PTA plant, selecting appropriate valves is crucial for handling various challenging conditions such as high temperatures, high pressures, corrosive environments, and slurry media. Plug valves and ball valves are preferred for their favorable flow characteristics, sealing performance, and low maintenance requirements. For high-temperature, high-pressure environments, flanged check valves offer better reliability. Special attention must be given to valve material selection and structural design to ensure durability and efficiency.

FAQs

1. What factors should be considered when selecting valves for slurry systems in a PTA plant?
   • Valves should have straight flow channels, be full-bore, and resistant to clogging. Plug valves and ball valves are preferred.
2. Why are flanged check valves preferred in high-temperature, high-pressure environments?
   • Flanged check valves provide better sealing reliability due to their resistance to thermal expansion and contraction compared to wafer connection check valves.
3. What is the significance of low leakage structures in valve selection for PTA plants?
   • Low leakage structures prevent corrosive media from damaging surrounding equipment and ensure better sealing performance.
4. How are tank bottom discharge valves designed to prevent clogging?
   • They use a straight-through structure, adjustable design, and short stroke plunger valve discs to ensure reliable sealing and reduce crystallization and clogging.
5. What are the general requirements for selecting valves in high-pressure slurry systems?
   • Valves should have low resistance drop, reliable sealing, and be able to handle both high-pressure and slurry media conditions.