Case Study 5: Hydrofluoric Acid

Problem: Chemical attack of HF

A large specialty chemical company had experienced multiple equipment failures over a period of several years in a reaction process involving an aqueous hydrogen fluoride (HF) solution. The reactor vessel and internal components were constructed of nonferrous high-nickel alloy. The alloy welds were being chemically attacked, and certain components, such as the agitator experienced life cycles as short as 3 weeks. The extremely high cost of downtime and maintenance over a sustained period drove the process engineers to seek alternative solutions.

Solution: E-CTFE lining system

The search led to fluoropolymer coating and lining systems. The application required a system that could withstand temperatures to 230°F (110°C) operate in a moderately abrasive aqueous HF slurry, and be chemically resistant to the non-HF components of the solution. Initial immersion tests in the chemical solution demonstrated that ethylene chlorotrifluoroethylene (ECTFE), a partially fluorinated fluoropolymer was resistant to the chemical attack. ASTM C 868 laboratory cell tests further confirmed that ECTFE in a bonded sheet lining system was resistant to both chemical attack and permeation in this environment.

As a result of these tests, the customer ordered a new reactor system consisting of ¼ in. (6-mm) carbon steel shell with a 90-mil (2-mm) ECTFE fabric-backed sheet lining. The lining was bonded to the shell with an elastomeric adhesive. The agitator shaft and blades were made of type 316 stainless steel, with a 40-mil ECTFE coating applied using a special primer and electrostatic powder spray process. The reactor system was purchased at a fraction of the cost paid for the corrosion-resistant alloy system that it replaced.

Benefit: Lower cost and longer life cycle

The new fluoropolymer-lined reactor ran continuously for 6 months before being shut down for normal preventive maintenance. Inspection of the reactor interior revealed no perceptible degree of chemical attack, blistering, discoloration, cracking, or abrasion damage. The agitator was removed and thoroughly inspected. The results showed it likewise was in a “like new” condition, with no chemical attack, delamination, discoloration, or abrasion damage. The system was put back into service and has continued to operate without failure. The customer subsequently purchased an identical fluoropolymer-lined reactor system for another plant.