How Does a PTFE Heat Exchanger Withstand Sulfuric Acid Catalyst Slurry in Alkylation Unit Heating?

The Acid Environment for Alkylation
In petroleum alkylation units , isobutane is reacted with light olefins in the presence of a sulfuric acid catalyst to produce high-octane components for gasoline . Acid catalyst is passed thru the reactor at a concentration of 85-98% and carries suspended acid soluble oils, tars and sludge particles. The mixture is somewhat corrosive and slightly abrasive. This aggressive fluid is attacking the heat exchanger, which is controlling the reaction temperature.

Metallic heat exchangers, even with high-alloy materials such as Hastelloy, have limited service life in this climate. The acid assaults the metal surface and a coating of corrosion product is formed that is then degraded by the suspended particles. The acid attacks new metal exposed by corrosion. The erosion-corrosion cycle promotes the loss of metal. Service life is usually 2 to 5 years; replacements are frequent.

PTFE is resistant to sulfuric acid at any concentration and at all temperatures up to 260°C. The homogeneous polymer doesn't have any corrosion product layer to wear down. The surface resists chemical damage and mechanical attack. The combined erosion-corrosion mechanism which destroys metallic exchangers does not take place on PTFE.

The Synergy of Erosion-Corrosion of Metal
In acid slurry treatment on a metallic surface, the damage progresses via a synergistic cycle. The acid attacks the metal surface, producing a coating of corrosion products – metal sulfates, oxides and hydrates. This layer is more flexible and less adhesive than the metal beneath it. The abrasive particles in the slurry abrade the corrosion product layer, revealing fresh metal. The new metal corrodes fast and the new layer of corrosion product is again degraded.

This cycle enhances the corrosion rate and the erosion rate. The overall rate of metal loss may be 5 to 20 times higher than the corrosion rate in acid without particles or the erosion rate in non-corrosive slurry.

PTFE will not corrode. No corrosion product layer is present. The acid encounters a chemically inert surface. The abrasive particles can induce slow mechanical wear yet the wear is not just uniform but predictable - no synergistic amplification. The wall of the tube is gradually worn away by pure mechanical erosion, at a rate measured in microns each year.

Alkylation Service ParameterPTFE Hastelloy C-276
Acid corrosion rate (particle-free) 0.05-0.15 mm/year 0
Formation of corrosion product layer Yes No
Corrosion product layer erosionYes (accelerates metal loss) N/A
Erosion-corrosion rate combined 0.25-0.75 mm/year 0.005-0.02 mm/year (mechanical wear only)
Service life (years) 2-5 15+
The advantage of Sludge Handling
The corrosion behavior is further complicated by the tendency of acid soluble oils and sludge present in the alkylation acid to deposit on the heat exchanger surfaces, forming a fouling layer that acts as insulation to the tubes. Corrosion product coatings on metallic surfaces create a rough high energy surface to attract sludge deposition. The muck builds up, hardens, and must be removed mechanically.

PTFE has a smooth surface with low surface energy making it difficult for sludge to adhere to. The sludge particles glide over the tube surface without adhering. Any little muck that does get deposited is simply washed away. No periodic sludge removal, therefore the surface remains clean and heat transfer remains stable.

The Process Compatibility Bonus
PTFE is unaffected by sulfuric acid over the entire spectrum of alkylation acid compositions-from fresh acid at 98% to spent acid at 85% with dissolved hydrocarbons and sludge. The material is not catalytic to acid-hydrocarbon reactions. It does not release metal ions that may interfere with the alkylation process. The PTFE exchanger provides a chemically inert element to a process where undesired catalysis can influence product quality.

Conclusion
PTFE heat exchangers are used for heating sulfuric acid catalyst slurry in alkylation units and provide superior service by reducing the erosion-corrosion synergy that promotes metal loss in metallic exchangers. The chemically inert surface does not have a layer of corrosion products to degrade. That smooth, low-energy surface resists attachment of the sludge. The service life of 15+ years versus 2-5 years for Hastelloy C-276. Consequently, the lower replacement frequency and reliable heat transmission make PTFE the better choice economically for alkylation acid operation.

Engineering help for PTFE heat exchanger specification in alkylation and acid sludge applications is provided with submission of acid composition, sludge content, operating temperature and current equipment service life.