In Which Chemical Services Does a Graphite Heat Exchanger Outperform a PTFE Unit?

Both graphite and PTFE are the champions in the corrosive heat transfer, yet both have their strong points. Knowing the services where graphite outperforms PTFE will prevent costly misapplications. PTFE is a great chemical resistant and mechanically flexible material but graphite has a superior thermal conductivity and high temperature resistance, making it the material of choice for a narrow range of corrosive activities. Graphite vs PTFE heat exchangers are well understood and engineers can choose the most reliable and cost-effective solution.

Hot Sulphuric Acid (70–98%) - The Fortress of Graphite
One of the most aggressive industrial chemicals is concentrated hot sulphuric acid. Graphite (impregnated with phenolic or PTFE resin, impermeable type) performs well in this environment up to 200 °C. The material is resistant to both oxidation and dehydration attack and maintains structural integrity even at high concentration.

While PTFE is limited to about 110°C for sustained duty in sulphuric acid. Above this temperature, PTFE weakens and loses its mechanical strength. Also the thermal conductivity gap becomes crucial. Efficient heat transfer is required for cooling or heating concentrated acid to avoid localised overheating or undercooling which can accelerate corrosion or cause product damage.

In applications such as the dilution of sulphuric acid, inter-stage cooling of contact plant or acid concentration units, graphite heat exchangers are typically the only practicable non-metallic solution. They have great thermal conductivity and enable compact designs. Their corrosion resistance is stable for years of use.

Hydrochloric Acid, Organic Contaminated
Hydrochloric acid is very corrosive to most metals in any quantity. Both graphite and PTFE are well tolerated by pure HCl. However, if the acid contains organic contaminants, such as chlorinated solvents, oils or reaction byproducts, graphite is frequently a preferable choice.

Graphite resists HCl up to its boiling point (110 °C for azeotropic strength) and is unaffected by trace organics. PTFE likewise resists these organics but the disadvantage of lower heat conductivity of PTFE. For a given duty, the heat transfer area of a PTFE exchanger is much bigger. Where the process fluid contains sticky organic substances, the longer tube lengths and larger shell of a PTFE unit increase the likelihood of fouling and difficulty of cleaning. Graphite is offered in compact block or shell-and-tube form, which minimises residence time and the surface area for deposition.

In practice, for HCl services with moderate to high heat loads, graphite offers a smaller, more responsive exchanger that is easier to clean and maintain.

Services Requiring High Rates of Heat Transfer
Graphite has a thermal conductivity of 80–120 W/m·K, several hundred times more than that of PTFE (0.25 W/m·K). Graphite is the material of choice where compactness and great thermal efficiency are critical. Benefit is maximal in:

High-flow liquid cooling or heating - The overall heat transfer coefficient for graphite liquid-liquid exchanger is normally 600–1200 W/m2·K whereas for PTFE it is 200–400 W/m2·K. The distinction is that a graphite unit might be 2 to 5 times smaller for the same duty.

Condensation of corrosive vapours - High tube-side film coefficients are required for condensing activities. The low conductivity of PTFE is a serious bottleneck due to the film resistance of the condensing liquid. Graphite will condense effectively, frequently in less than one third of the area required for a PTFE unit.

Fast batch heating or cooling - For batch operations where cycle time is crucial, the low thermal mass and strong thermal conductivity of graphite allow for rapid temperature ramps. The sluggish responsiveness of PTFE also adds to cycle time.

The compactness of graphite is a deciding factor in applications where space is at a premium such as chemical reactor skids, pharmaceutical cleanrooms or retrofits in existing buildings.

When PTFE is Better than Graphite (Acknowledgement)
Any fair comparison must acknowledge services for which PTFE is superior. PTFE is better than graphite in below:

Service Why You Should Choose PTFE
Strong oxidising acids (e.g., concentrated nitric acid, chromic acid) Under oxidation circumstances the resin impregnation is attacked causing loss of corrosion resistance in impermeable graphite. PTFE is essentially non-oxidizing.
Hot strong caustics (e.g. 50% NaOH above 100 °C)Hot alkalis attack graphite. PTFE resists caustics up to its temperature limits.
Oleum (fuming sulphuric acid containing free SO₃)Graphite reacts with free SO 3 . PTFE is stable .
Systems with high vibrations or water hammersGraphite is brittle therefore it cracks . PTFE is flexible so it takes the mechanical shock .
Large or abrasive substances in fluids Graphite passageways plug or degrade. PTFE tubes can bend and go through particles.
Graphite will react with oleum, hot strong caustics or oxidising acids e.g. concentrated nitric. For these services, PTFE or other materials (e.g. tantalum, silicon carbide) should be investigated.

Summary Table: Guidance for Decision
A brief guide to choosing graphite or PTFE for chemical duty is shown in the table below:

Chemical Service Graphite Performance PTFE Performance Materials Preferred
Hot H2SO4 (70-98%, 80-200oC)Excellent Temperature limited