Typically, heat exchangers are fabricated from steel, tantalum and other metals, fluoropolymers, glass or graphite. However, when dealing with diluted sulfuric acid, the list of appropriate construction materials is greatly reduced to fluoropolymers, glass, graphite and tantalum.
Along with how well they handle corrosive sulfuric acid, these materials may be greatly affected by changes in temperature, concentration level or the pH of the process stream. Also, some materials are more inherently susceptible to thermal and mechanical shock. Finally, specific applications may have their own set of process and equipment issues that can detrimentally affect different materials.
This overview describes the advantages and disadvantages of various heat exchanger construction materials and offers criteria for selecting one heat exchanger versus another for diluted sulfuric acid applications.
Tantalum Heat Exchangers
Metal heat exchangers are known for their durability. For diluted sulfuric acid applications in which a metal heat exchanger is an option, tantalum is the metal of choice. It provides exceptional chemical resistance that results largely from a protective oxide film that forms on the metal’s surface. However, along with this key advantage, there are disadvantages with tantalum that need to be considered when selecting it for a heat exchanger.
Advantages. Tantalum exchangers offer excellent corrosion resistance to diluted sulfuric acid. They are available in many sizes and shapes. They are able to withstand high pressure, temperature and flow rates and are less susceptible to damage caused by thermal shock or differential expansion. Tantalum also can be fashioned into large diameter tubes that aid in minimizing pressure drop and the potential for plugged tubes.
Disadvantages. Tantalum’s protective oxide film can be damaged or destroyed by changes in acid concentration, temperature or the pH of the process stream. Without that protective film, metal corrosion may cause leaks or ruptures to occur or make the tantalum heat exchangers susceptible to fouling. Tantalum deposits may partially block tubes within the exchanger or affect the quality of the process stream. Over time, both corrosion and fouling may lead to a measurable reduction in heat exchanger performance, requiring either specialized welding repairs or a complete heat exchanger replacement.
Glass Heat Exchangers
The process industry has used glass heat exchangers for years as a corrosion-resistant option for handling sulfuric acid. Glass heat exchangers offer an exceptional degree of corrosion resistance. They also address many of the common process issues associated with sulfuric acid such as changes in concentration levels and pH.
Advantages. Glass heat exchangers are available in several shell options. Among the most common are those manufactured with glass tubes and a carbon steel shell. With the corrosive acid stream contained within the tubes, a metal shell increases the pressure limits of the shell-side fluid. A glass shell-and-tube heat exchanger can withstand corrosive process streams on both the tube side and shell side.
The glass heat exchanger design allows damaged tubes to be removed relatively easily and replaced. The smooth glass tube surface resists fouling, and any permeation through the glass is minimal. Resistance to fouling and permeation can increase the purity of a sulfuric acid process stream considerably.
Disadvantages. Along with their advantages, glass heat exchangers have disadvantages. The maximum thermal shock temperature range for a glass exchanger is highly dependent on the exchanger’s specific design. This range represents the maximum instantaneous temperature difference to which glass can be subjected without risk. The cyclic temperature service of sulfuric acid dilution can result in high induced tensile stresses, which can lead to tube breakage. Safeguards against thermal shock as well as mechanical damage from water hammer and steam hammer also must be carefully considered with all glass heat exchanger installations.
Along with operational risks, maintenance risks need to be taken into consideration. The cyclic nature of sulfuric acid dilution can cause individual glass tubes to fail prematurely or to fail throughout an entire heat exchanger. Routine maintenance can leave a glass heat exchanger vulnerable to damage from the metal tools used due to the fragile nature of glass. That fragility further limits the size of glass heat exchangers and places special handling and production requirements on the heat exchangers.
Graphite Heat Exchangers
Impervious graphite heat exchangers can be manufactured in three forms: shell and tube; cubic or rectangular block; or block-in-shell designs. In diluted sulfuric acid applications, each type of design must be carefully considered. Regardless of design, the advantages and disadvantages for graphite heat exchangers are similar.
Advantages. Graphite heat exchangers are available in a range of sizes with some models providing higher pressure capabilities. The low thermal expansion coefficient of graphite provides high resistance to thermal shock. Pertaining to industrial diluted sulfuric acid applications, graphite heat exchangers are reasonably resistant to corrosion and fouling, making them suitable for process applications where purity is a requirement.
Disadvantages. Graphite exchangers have notch sensitivity, which is a measure of the reduction in strength of a metal caused by the presence of stress from an intersection or notch. In effect, graphite exchangers are fragile and can be easily damaged by mechanical or hydraulic shock. Special flexible couplings or expansion joints are recommended for all process nozzle connections to avoid stress on the graphite nozzle liners.
Great care must be taken to avoid tensile loads on the tubes. Tube-to-tube sheet and tube splicing joints are cemented together, and thermal expansion differences between the adhesive layer and bulk graphite can lead to premature failures. Graphite exchangers also have potential process stream carbon contamination issues.
Fluoropolymer Heat Exchangers
Fluoropolymer heat exchangers are well suited for handling all concentrations of diluted sulfuric acid. They are resistant to corrosion and fouling and are a good choice when dealing with purity, durability and efficiency concerns. As with other heat exchanger construction materials, the advantages and disadvantages of fluoropolymers need to be considered before selecting them for a particular application.
Advantages. Fluoropolymer heat exchangers are inert to all concentrations of diluted sulfuric acid. They can significantly increase the purity of process streams in a range of applications. Acid contamination, if present, will not affect the corrosion resistance performance of the heat exchanger.
Fluoropolymer heat exchangers offer excellent durability. They are not susceptible to thermal or mechanical shock. The nonstick characteristics of fluoropolymers help prevent harmful scaling or fouling. If necessary, tube repairs can be made quickly on-site without removal of the heat exchanger.
Disadvantages. Along with their numerous advantages, especially in highly corrosive acid applications, fluoropolymer heat exchangers come with a few disadvantages. Fluoropolymers are natural insulators. In order to meet the application’s heat transfer requirements, fluoropolymer heat exchangers are manufactured with smaller diameter tubes. These tubes can plug if a process includes large suspended solids. Design considerations also may impose restrictions on operational pressures, temperatures or flow rates, especially when high flow rates of sulfuric acid are involved.
In conclusion, in any diluted sulfuric acid application, there are three main concerns: durability, purity and efficiency. Specific applications have their own set of process and equipment issues that can result in detrimental effects on different types of heat exchanger construction materials.
Thermal shock, mechanical shock, differential expansion and corrosion all can affect heat exchanger performance. Corrosion can cause fouling, scaling or leaks. If a heat exchanger material of construction is significantly affected by these process issues, its durability may be compromised. This could result in costly maintenance downtime or premature failure.
Efficiency should not simply be considered as the thermal performance of a heat exchanger. There are two types of efficiency that need to be addressed when choosing heat exchanger fabrication material: thermal efficiency and mechanical efficiency. A proper thermal design will provide the necessary surface area required to meet process requirements. Mechanical efficiency should be viewed as the ability of the heat exchanger to perform with minimal required maintenance.
Overall, proper material selection will notably reduce heat exchanger operating costs by increasing durability, reducing corrosion and fouling, and improving mechanical and thermal efficiency. Proper material selection can avoid costly downtime and greatly reduce process contamination. Finally, heat exchanger service life can be greatly extended if a careful analysis of process equipment issues is made prior to heat exchanger material selection.