The Chemical Process Industries (CPI) face many challenges in the containment of highly corrosive products and intermediates, both in processing facilities and in the treatment of process wastes. Often the combination of temperatures and pressures severely limits the service life of conventional systems. Successful containment needs to be cost effective, safe and environmentally responsible.
These requirements create the need for more corrosion resistant equipment and piping suitable not only for aggressive individual chemicals, but also for multi-component mixtures, whose compositions may vary with time and over a wide range of pH. Historically, metals have been used to solve most corrosion problems, however, some current process conditions exceed the capabilities of even the most expensive and exotic metals.
As an alternative, fluoropolymers can provide successful containment facilities in many cases.Fluoropolymers are highly corrosion resistant to a wider range of chemicals at varying concentrations and pH's.
Permeation Through Fluoropolimers
Permeation is the transport of small molecules through the plastic liner. Permeation is a combination of absorption, solution, diffusion & desorption mechanisms. It is a function of several variables including the properties of the permeating species (i.e., molecular size, solubility parameters, concentration), properties of the plastic (i.e., crystallinity, density, thickness, thermal history), and operating parameters (i.e., temperature-pressure gradients and cycling). Permeation data published in most literature refers to thin coatings or films and should not be applied to plastic-lined products.
All products lined with fluoropolymers, such as PTFE, PFA, FEP, ETFE and ECTFE are subject to some permeation for certain fluids. Normally PTFE lined products in conjunction with the fluids having permeation in PTFE are substituted by PVDF to avoid permeation phenomenon.
Fluoropolymer Selection Guide
| ||PTFE ||PFA ||FEP ||ETFE ||ECTFE ||PVDF |
|Theoretical max.Temp. ||260 oC ||260 oC ||205 oC ||150 oC ||150 oC ||135 oC |
|Recommended max. Temp. ||200 oC ||200 oC ||160 oC ||For diff. concentrations and pH values Recommended temp. Varies widely. Refer corrosion resistance chart. |
|Solvents ||No known solvents ||No known solvents under ordinary conditions ||Some fluorinated Solvents ||Common industrial Ketones |
|Corrosion resistance ||Virtually all chemicals except fluorine & related compounds and violent reducingagents like metallic sodium and molten alkali metals ||Refer detailed corrosion resistance chart to ensure chemical compatibility. |
TEFLON is a registered trademark of Dupont, USA for PTFE, FEP & PFA fluoropolymers.
TEFZEL is a registered trademark of Dupont, USA for ETFE fluoropolymer.
Selecting the right fluoropolymer material for hazardous or hard to handle fluids is the most essential element in the process industry. Certain special conditions may affect the performance or service life of the fluoropolymer. For the proper selection of plastic lined products, the following information should be considered.