The Mixed Acid Compositions That Define the Boundaries
Mixed acid recovery systems handle several different acid combinations, and the material selection depends on which combination is present.
Nitric-hydrofluoric mixtures. These are used for stainless steel pickling and titanium etching. The nitric acid provides the oxidizing potential, and the hydrofluoric acid provides the fluoride activity. The mixture is aggressive to most metals and to many ceramics. PTFE resists the mixture at all concentrations and temperatures within its range. Silicon carbide resists the mixture at temperatures up to 80°C, but the fluoride attacks the silicon carbide at higher temperatures.
Sulfuric-hydrofluoric mixtures. These are used for aluminum etching and glass etching. The sulfuric acid provides the acid strength, and the hydrofluoric acid provides the fluoride activity. PTFE resists the mixture at all concentrations and temperatures. Silicon carbide resists the mixture at temperatures up to 60°C, but the fluoride attacks the silicon carbide at higher temperatures.
Nitric-sulfuric mixtures. These are used for metal pickling and passivation. The mixture is strongly oxidizing, and the oxidizing potential increases with the nitric acid concentration. PTFE resists the mixture at all concentrations and temperatures. Silicon carbide resists the mixture at temperatures up to 100°C, but the oxidizing potential can attack the grain boundary phase in the silicon carbide.
The Thermal Conductivity Gap
Silicon carbide has a thermal conductivity of approximately 120 W/m·K, compared to 0.25 W/m·K for PTFE. The ratio is approximately 480:1.
For a heat exchanger delivering 60 kW in a mixed acid recovery system at 70°C, a silicon carbide unit requires approximately 3 m² of surface area, based on an overall heat transfer coefficient of 1,000 W/m²·K and a log mean temperature difference of 20°C. A PTFE unit for the same duty requires approximately 15 m² of surface area, based on an overall coefficient of 200 W/m·K and the same temperature difference.
The PTFE unit is 5 times larger than the silicon carbide unit. The larger size increases the capital cost, the floor space, and the fluid volume. The lower heat flux reduces the tube wall temperature, which is beneficial for the process, but the larger surface area also provides more surface for deposits and requires more time to clean.
The Fluoride Attack on Ceramics
The fluoride in mixed acid recovery systems attacks ceramics through a mechanism that is different from the attack on metals. The fluoride reacts with the silicon dioxide or the alumina in the ceramic, forming soluble fluoride compounds that dissolve the ceramic. The attack rate depends on the fluoride concentration, the temperature, and the specific ceramic composition.
Silicon carbide has better fluoride resistance than alumina-based ceramics because the silicon carbide grains are bonded with a silicon nitride or a silicon carbide matrix that is more resistant to fluoride. However, the grain boundary phase can be attacked by fluoride at elevated temperature, and the attack weakens the ceramic and eventually causes failure.
PTFE is resistant to fluoride at all concentrations and temperatures within its service range. The carbon-fluorine bonds are not attacked by fluoride, and the polymer structure is not degraded by the chemical environment. This is the primary reason PTFE is specified for mixed acid recovery systems with high fluoride concentrations.
Comparing the Two Materials in Mixed Acid Recovery
The following table summarizes the performance and cost characteristics of PTFE and silicon carbide heat exchangers in mixed acid recovery service at 70°C with 10% nitric acid and 3% hydrofluoric acid.
| Characteristic | Silicon Carbide Heat Exchanger | PTFE Heat Exchanger | Advantage |
|---|---|---|---|
| Thermal conductivity | 120 W/m·K | 0.25 W/m·K | Silicon carbide |
| Overall heat transfer coefficient | 1,000 W/m²·K | 200 W/m²·K | Silicon carbide |
| Surface area required for 60 kW | 3 m² | 15 m² | Silicon carbide |
| Chemical resistance to nitric-hydrofluoric mixture | Good at 70°C | Excellent | PTFE |
| Chemical resistance to fluoride | Moderate | Excellent | PTFE |
| Maximum operating temperature | 80°C (with fluoride) | 260°C | PTFE |
| Mechanical strength | High | Moderate | Silicon carbide |
| Thermal shock resistance | Low | High | PTFE |
| Mechanical impact resistance | Low | High | PTFE |
| Repairability | Limited | Good | PTFE |
| Service life in mixed acid service | 4–6 years | 10–15 years | PTFE |
| Capital cost for 60 kW duty | $48,000 | $42,000 | PTFE |
| Installation cost | $6,000 | $4,000 | PTFE |
| Annual maintenance cost | $5,000 | $2,000 | PTFE |
| Five-year total cost | $109,000 | $72,000 | PTFE |
The five-year total cost values assume a production value of $600 per hour and include the capital cost, the installation cost, and the maintenance cost over five years.
When Ceramic Is the Better Choice
Ceramic is the better choice in several specific applications.
Low-fluoride service. When the fluoride concentration is below 1%, the ceramic fluoride attack rate is low, and the service life is extended. In this case, the higher thermal conductivity of silicon carbide reduces the required surface area and the capital cost.
Space-constrained installations. When the available space is severely limited, the smaller silicon carbide unit may be the only feasible option. The PTFE unit would require 5 times the surface area, which may not fit in the available space.
High-temperature service without fluoride. When the mixed acid contains no fluoride and the temperature is above 100°C, silicon carbide retains its mechanical strength and chemical resistance, while PTFE softens and loses strength above 200°C.
High-purity service. Silicon carbide does not leach organic compounds into the process fluid, which makes it suitable for high-purity applications where the organic leaching from PTFE is a concern.
Abrasive service. Silicon carbide has higher hardness and wear resistance than PTFE, which makes it suitable for service with suspended solids that could abrade the heat exchanger surface.
When PTFE Is the Better Choice
PTFE is the better choice in several specific applications.
High-fluoride service. PTFE is resistant to fluoride at all concentrations, while silicon carbide is attacked by fluoride at elevated temperature.
Thermal cycling service. PTFE has higher thermal shock resistance than silicon carbide, which makes it suitable for service with frequent temperature changes.
Mechanical impact service. PTFE is more resistant to mechanical impact than silicon carbide, which makes it suitable for installations where the heat exchanger may be subject to vibration or handling.
Repairable service. PTFE heat exchangers can be repaired on site by a qualified technician, which reduces the downtime and the production loss.
Large heat load service. For large heat loads, the lower capital cost of PTFE becomes more significant, and the larger surface area is less of a constraint.
Design Features That Extend Service Life
Several design features extend the service life of both PTFE and ceramic heat exchangers in mixed acid recovery service.
Lower operating temperature. Reducing the operating temperature reduces the fluoride attack on the ceramic and the diffusion of the acid into the PTFE.
Lower fluoride concentration. Reducing the fluoride concentration reduces the attack on the ceramic. The fluoride concentration should be monitored continuously and maintained below the threshold for the specific ceramic material.
Flow velocity control. Maintaining a moderate flow velocity reduces the boundary layer thickness and the concentration of fluoride at the tube surface.
In-place cleaning provisions. Connections for cleaning solution supply and return allow the heat exchanger to be cleaned without disassembly, which reduces the downtime and the production loss.
Temperature monitoring. Temperature sensors at the heat exchanger inlet and outlet allow the heat transfer performance to be monitored, which allows early detection of fouling or degradation.
Specification Parameters for Mixed Acid Recovery
Equipment engineers evaluating PTFE and ceramic heat exchangers for mixed acid recovery should confirm the following parameters with the supplier.
Mixed acid composition. Specify the concentrations of nitric acid, sulfuric acid, hydrofluoric acid, and any other components.
Operating temperature. Specify the maximum operating temperature and the expected temperature profile.
Fluoride concentration. Specify the maximum fluoride concentration and the monitoring method.
Suspended solids. Specify the concentration and particle size of any suspended solids in the acid.
Heat load and surface area. Specify the heat load and the required surface area for each material option.
Space constraints. Specify the available space and the maximum allowable footprint.
Maintenance capability. Specify the maintenance capability of the facility, including the ability to repair the heat exchanger on site.
Service life requirement. Specify the expected service life and the replacement schedule.
Matching Material Selection to Mixed Acid Service Conditions
The selection between PTFE and ceramic heat exchangers in mixed acid recovery depends on the fluoride concentration, the temperature, the space constraints, and the service life requirement. Ceramic has a higher thermal conductivity and requires less surface area, but it is susceptible to fluoride attack and thermal shock. PTFE has a lower thermal conductivity and requires more surface area, but it is resistant to fluoride and thermal shock.
The selection decision should be based on a facility-specific analysis that incorporates the capital cost, the installation cost, the maintenance cost, and the production loss exposure. A simple spreadsheet model with these cost components provides the information needed to make the decision.
For mixed acid recovery facilities evaluating heat exchanger options, the material selection should be based on the total cost of ownership over the expected service life, not just the thermal performance or the capital cost. A ceramic unit with a higher thermal conductivity may have a higher lifetime cost if the fluoride concentration is high and the unit requires frequent replacement.
Equipment engineers and procurement specialists evaluating PTFE heat exchanger specifications for mixed acid recovery or other corrosive service can request a design review covering mixed acid composition, operating temperature, fluoride concentration, suspended solids, and space constraints to confirm the material selection is matched to the actual service conditions and cost structure of the facility.
