The Oleum Environment
Fuming sulfuric acid-oleum-contains dissolved sulfur trioxide in sulfuric acid, with free SO₃ concentrations of 20-65%. At 150°C, it is one of the most aggressive industrial chemicals. It is strongly oxidizing, strongly dehydrating, and attacks most metals and polymers.
Tungsten is one of the few metals with useful resistance to hot oleum. Its passive film-WO₃-is stable in strong oxidizing acids. Tungsten heat exchangers are used in specialized oleum service where the material's high density (19,300 kg/m³) and high cost are justified by the process conditions.
PTFE is resistant to oleum at all concentrations and temperatures up to 260°C. The comparison between tungsten and PTFE in this environment is between the most resistant metal and an unconditionally inert polymer.
The Tungsten Passive Film Limits
Tungsten resists oleum through a WO₃ passive film that forms spontaneously in oxidizing environments. The film is stable in sulfuric acid and oleum at temperatures up to approximately 180°C, depending on the SO₃ concentration. Above this temperature, or at certain SO₃ concentrations, the film can dissolve, exposing the underlying metal to rapid attack.
The passive film is also sensitive to impurities in the oleum. Trace water (from incomplete dehydration) can hydrolyze the WO₃ to soluble tungstic acid species. Trace chloride can cause pitting. The film requires continuous maintenance by the oxidizing environment. Any process upset that changes the SO₃ concentration or introduces impurities can destabilize the film.
| Corrosion Parameter | Tungsten | PTFE |
|---|---|---|
| Corrosion protection mechanism | WO₃ passive film | Inherent C-F bond stability |
| Maximum service temperature in 30% oleum (°C) | ~180 | 260 |
| Sensitivity to SO₃ concentration variation | Moderate | None |
| Sensitivity to water/impurity excursions | High (film destabilization) | None |
| Corrosion rate during stable operation (mm/year) | < 0.02 | 0 |
| Corrosion rate during process upset (mm/year) | 0.5-2.0 (film breakdown) | 0 |
| Material density (kg/m³) | 19,300 | 2,200 |
| Material cost (relative) | 80-120× | 1× |
The Excursion Survivability
A process upset in an oleum plant-a drop in SO₃ concentration from 30% to 15% due to air in-leakage, or a temperature spike from 150°C to 190°C from a control failure-can destroy the tungsten passive film. The film, once destabilized, may not repassivate when normal conditions are restored. The exchanger suffers irreversible corrosion damage.
PTFE has no passive film to destroy. An upset of any magnitude-SO₃ concentration swing, temperature excursion, impurity ingress-leaves the material unchanged. When normal conditions return, the PTFE surface is identical to its pre-upset state. This survivability is as important as the steady-state corrosion rate in determining the practical reliability of the equipment.
The Cost and Weight Comparison
Tungsten heat exchangers are extraordinarily expensive-80-120 times the cost of a PTFE equivalent. Tungsten is one of the densest engineering materials. A tungsten exchanger is heavy, requiring substantial structural support.
PTFE exchangers cost a fraction as much and weigh less than one-eighth as much. Where the process temperature is within PTFE's 260°C limit, PTFE is the economically and operationally superior choice. Tungsten is reserved for applications where the temperature exceeds PTFE's limit, or where the mechanical strength of metal is required.
Summary
PTFE heat exchangers are unconditionally resistant to fuming sulfuric acid (oleum) at 150°C, while tungsten relies on a WO₃ passive film that can be destabilized by process upsets. PTFE provides upset survivability that tungsten cannot match, at a small fraction of the cost and weight. Tungsten is limited to applications exceeding PTFE's temperature limit.
Engineering support for PTFE heat exchanger specification in oleum and strong acid service is available upon submission of acid composition, operating temperature, upset scenarios, and space constraints.
