Achieve maximum productivity of the high-pressure stripper

The Challenge

The high-pressure (HP) stripper in Stamicarbon CO₂ stripping plants is a vertical falling film evaporator wherein a urea synthesis stream is brought in contact with a counter-current stream of CO₂ gas flowing upwards in the heat exchanger tubes.

The liquid film is created by installing ferrules on top of all vertical heat exchanger tubes, consisting of a liquid divider and a gas tube. They ensure the equal distribution of the urea/carbamate solution over all tubes after the liquid enters the top of the stripper. 

Liquid dividers create a uniform falling liquid film down to the heat exchanger tubes.

This film is created by three equally spaced liquid holes with a specific hole diameter. The CO₂ gas introduced at the bottom of the heat exchanger tubes together with the stripped gas (CO₂ and NH₃ mixture) is expelled via the gas tubes.  

The HP stripper is designed to remain in operation for years, but cross-cut end attack, the type of corrosion common for duplex stainless steels, presents a challenge for liquid dividers. It increases the hole size thus increasing the liquid load in each heat exchanger tube, which over time could impair the HP stripper efficiency, as a result preventing the stripper, and subsequently the plant, from achieving maximum productivity.

Then, how to increase the effective lifetime of these liquid dividers? 

Our Solution: Safurex^® Degree°

To address the issue of cross-cut end attack in liquid dividers, we developed Safurex^® Degree° stainless steel. This material is fabricated via the Hot Isostatic Pressing (HIP) method, which results in an isotropic and fine-grained microstructure, improving corrosion resistance and besides that enhancing the material’s mechanical properties at low temperatures. 

We also observed that Safurex^® Degree° outperforms the “standard” Safurex^® (Infinity∞) in tests in oxygen-free ammonium carbamate solution.

The first liquid dividers made of Safurex^® Degree° material installed in an HP stripper showed hardly any cross-cut end attack corrosion in the liquid divider holes after more than seven years on-stream time. 

The following advantages of using Safurex^® Degree° steel for liquid dividers are evident and include:

• Extended lifetime of liquid dividers to 20 years
• Improved resistance to corrosion, especially cross-cut end attack 

Safurex^® Degree° material can be used for the manufacturing of other parts that require either extreme corrosion resistance and/or low MDMT (minimum design metal temperature) requirements, which sometimes are specified for high-pressure accessories, such as HP Safurex^® ejectors, Safurex^® control valves, Safurex^® check valves and Safurex^® safety valves. 

Our Safurex^® liquid dividers are based on years of experience and design studies and will be customized based on your process conditions. Contact us to learn more.