A chemical manufacturer had (6) 300-gal glass-lined reactors containing mixtures of hydrochloric acid (HCl), acetic acid (CH³COOH), and various other reactants. Operating temperatures were in the 200 to 275°F (93 to 135°C) range, and periodically the system was subjected to full vacuum conditions. Physical shocks frequently caused minor damage in the form of small chips in the glass lining. These chips routinely were repaired with tantalum alloy plugs. Concurrent with the repairs, production engineers often experienced unacceptable variations in the purity of the finished product. The tantalum alloy plugs were ultimately determined to be the contamination source.
A decision was made by the production management to keep spare glass-lined vessels on site and to completely replace damaged vessels rather than repair them with tantalum plugs. The heavy expense of changing out and relining the reactors each time the glass chipped eventually made this option unacceptable to management.
In search for a cost-effective alternative, fluoropolymer coating and linings were evaluated. The combination of aggressive chemicals, full vacuum conditions, temperatures >200°F and heavy physical stresses made this a particularly challenging problem.
One option with a history of success in similar environments was a high-build polyvinylidene fluoride (PVDF) dispersion laminate system. Rather than run an ASTM C868 cell test in the laboratory to test for chemical attack and permeation, the customer decided to test a system in actual operations. The glass lining was stripped from a reactor and agitator, and they were coated with a 40-mil PVDF dispersion laminate reinforced with carbon fiber cloth.
The PVDF laminate system was placed in service and ran for four months without chemical attack, permeation or failure caused by mechanical damage of the lining. Based on these results, the customer converted the remaining five glass-lined reactors and agitators to the carbon cloth-reinforced PVDF dispersion laminate. The cost of the PVDF coating system was significantly less than that of the equivalent glass-lined system, but the real payback has been from increased production and elimination of unscheduled downtime to change out reactors.
