The Chemistry That Attacks Titanium
Titanium resists sodium hypochlorite through the formation of a passive oxide film that is stable in neutral and alkaline solutions. The film is the reason titanium performs well in bleach service at moderate temperature and concentration. The film breaks down when the temperature rises above 70°C, when the concentration rises above 15%, or when the pH drops below 9.
The corrosion rate of titanium in hypochlorite service at 60°C and 12% hypochlorite is typically less than 0.05 mm per year. At 80°C and 15% hypochlorite, the rate increases to 0.3–0.8 mm per year. At 90°C and 15% hypochlorite, the rate can exceed 1.5 mm per year.
The corrosion typically initiates at the tube ends where the flow velocity is low and the temperature is high, and at the tube-to-tube sheet joints where the crevice geometry concentrates the corrosive species. The corrosion manifests as pitting, which penetrates the tube wall and causes a leak.
The Chemistry That Attacks PTFE
PTFE is resistant to sodium hypochlorite at all concentrations and temperatures within its service range. The carbon-fluorine bonds are not attacked by the hypochlorite or the caustic soda that stabilizes the bleach. The material does not corrode, and the service life is determined by mechanical factors rather than chemical attack.
The only degradation mechanism for PTFE in hypochlorite service is the diffusion of the hypochlorite and the caustic into the amorphous regions of the polymer. The diffusion is slow, and the effect on the mechanical properties is modest. The service life of a PTFE heat exchanger in hypochlorite service is typically 12–18 years.
The Thermal Conductivity Gap
The thermal conductivity of titanium is approximately 22 W/m·K, compared to 0.25 W/m·K for PTFE. The ratio is approximately 88:1.
For a heat exchanger delivering 40 kW in 12% hypochlorite at 60°C, a titanium unit requires approximately 3 m² of surface area, based on an overall heat transfer coefficient of 800 W/m²·K and a log mean temperature difference of 17°C. A PTFE unit for the same duty requires approximately 12 m² of surface area, based on an overall coefficient of 200 W/m·K and the same temperature difference.
The PTFE unit is 4 times larger than the titanium 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 Decomposition Problem at Elevated Temperature
Sodium hypochlorite decomposes at a rate that increases with temperature. At 60°C, the decomposition rate is approximately 2% per day. At 70°C, the rate is approximately 5% per day. At 80°C, the rate is approximately 10% per day. The decomposition generates chlorine gas and hydrochloric acid, which are corrosive to both materials.
The decomposition rate limits the practical operating temperature for both titanium and PTFE. At temperatures above 70°C, the decomposition rate is high enough that the hypochlorite concentration must be continuously replenished, and the chlorine gas must be vented. The material selection must account for the chlorine gas and the decomposition products.
Titanium is particularly susceptible to the hydrochloric acid that is generated by the decomposition. The acid lowers the pH and accelerates the titanium corrosion. PTFE is resistant to the hydrochloric acid and the chlorine gas, so the decomposition does not affect the PTFE service life.
Comparing the Two Materials in Hypochlorite Bleach Service
The following table summarizes the performance and cost characteristics of PTFE and titanium heat exchangers in hypochlorite bleach service at 60°C with 12% sodium hypochlorite.
| Characteristic | Titanium Heat Exchanger | PTFE Heat Exchanger | Advantage |
|---|---|---|---|
| Thermal conductivity | 22 W/m·K | 0.25 W/m·K | Titanium |
| Overall heat transfer coefficient | 800 W/m²·K | 200 W/m²·K | Titanium |
| Surface area required for 40 kW | 3 m² | 12 m² | Titanium |
| Chemical resistance to 12% hypochlorite at 60°C | Excellent | Excellent | Equal |
| Chemical resistance to 15% hypochlorite at 80°C | Poor | Excellent | PTFE |
| Chemical resistance to chlorine gas | Moderate | Excellent | PTFE |
| Maximum operating temperature | 70°C (practical) | 260°C | PTFE |
| Mechanical strength | High | Moderate | Titanium |
| Thermal shock resistance | Moderate | High | PTFE |
| Repairability | Limited | Good | PTFE |
| Service life in hypochlorite service | 5–8 years | 12–18 years | PTFE |
| Capital cost for 40 kW duty | $32,000 | $34,000 | Titanium |
| Installation cost | $4,000 | $3,500 | PTFE |
| Annual maintenance cost | $4,000 | $1,500 | PTFE |
| Five-year total cost | $76,000 | $55,500 | 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 Titanium Is the Better Choice
Titanium is the better choice in several specific applications.
Low-temperature service. At temperatures below 50°C, titanium resists hypochlorite without difficulty, and the higher thermal conductivity reduces the required surface area and the capital cost.
Low-concentration service. At concentrations below 10%, titanium resists hypochlorite at temperatures up to 60°C, and the higher thermal conductivity reduces the required surface area.
Space-constrained installations. When the available space is severely limited, the smaller titanium unit may be the only feasible option. The PTFE unit would require 4 times the surface area, which may not fit in the available space.
High-pressure service. Titanium has higher mechanical strength than PTFE and can withstand higher pressures. For hypochlorite service at pressures above 10 bar, titanium may be the only viable option.
When PTFE Is the Better Choice
PTFE is the better choice in several specific applications.
High-temperature service. At temperatures above 60°C, PTFE resists the decomposition products better than titanium, and the service life is longer.
High-concentration service. At concentrations above 12%, PTFE resists the hypochlorite and the caustic better than titanium.
Chlorine gas exposure. If the decomposition generates chlorine gas, PTFE resists the gas better than titanium.
Thermal cycling service. PTFE has higher thermal shock resistance than titanium, which makes it suitable for service with frequent temperature changes.
Long service life requirement. When the heat exchanger is expected to have a service life of 10 years or more, PTFE is the better choice because it resists the chemical attack over a longer period.
Design Features That Extend Service Life
Several design features extend the service life of both PTFE and titanium heat exchangers in hypochlorite bleach service.
Lower operating temperature. Reducing the operating temperature from 70°C to 50°C reduces the decomposition rate and the chemical attack on the heat exchanger material.
Lower concentration. Reducing the hypochlorite concentration from 15% to 10% reduces the chemical attack and the decomposition rate.
pH control. Maintaining the pH above 10 reduces the corrosion of titanium and the decomposition of the hypochlorite.
Flow velocity control. Maintaining a moderate flow velocity reduces the boundary layer thickness and the concentration of decomposition products 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.
Specification Parameters for Hypochlorite Bleach Service
Equipment engineers evaluating PTFE and titanium heat exchangers for hypochlorite bleach service should confirm the following parameters with the supplier.
Hypochlorite concentration. Specify the maximum concentration and the expected range.
Operating temperature. Specify the maximum operating temperature and the expected temperature profile.
pH. Specify the pH range and the control method.
Operating pressure. Specify the maximum operating pressure.
Heat load and surface area. Specify the heat load and the required surface area for each material option.
Decomposition rate. Specify the expected decomposition rate and the chlorine gas generation rate.
Space constraints. Specify the available space and the maximum allowable footprint.
Service life requirement. Specify the expected service life and the replacement schedule.
Matching Material Selection to Hypochlorite Service Conditions
The selection between PTFE and titanium heat exchangers in hypochlorite bleach service depends on the temperature, the concentration, the pH, and the service life requirement. Titanium has a higher thermal conductivity and requires less surface area, but it is susceptible to corrosion at elevated temperature and high concentration. PTFE has a lower thermal conductivity and requires more surface area, but it is resistant to hypochlorite at all concentrations and temperatures.
The selection decision should be based on a facility-specific analysis that incorporates the capital 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 hypochlorite bleach service 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 capital cost or the thermal performance. A titanium unit with a lower capital cost may have a higher lifetime cost if it requires frequent replacement.
Equipment engineers and procurement specialists evaluating PTFE heat exchanger specifications for hypochlorite bleach or other oxidizing service can request a design review covering hypochlorite concentration, operating temperature, pH, operating pressure, and service life requirements to confirm the material selection is matched to the actual service conditions and cost structure of the facility.
