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Get Acrobat Reader You will need Acrobat Reader to view pdf Handling Sulfuric Acid

Sulfuric acid is difficult to handle. But it must be handled because many manufactured articles depend in some way on sulfuric acid for their production. Most piping systems cannot handle all concentrations of sulfuric acid, especially oleum and multiple solutions over the full range of operating temperatures. Mild steel, stainless, alloys, plastic, fiber-reinforced plastic, glass and graphite all have something to offer but many have limitations.

Acid dilutions in plastic-lined pipe

One of the more common operations in handling sulfuric acid in lined piping is dilution to a lower strength acid. This is usually done with a mixing tee. There are usually no problems in doing this if the heat of dilution is taken into consideration during system design. Therefore, the following points should be considered in the design and installation of any system in which acid dilution will take place:

Physical property data

Freezing points, H2SO4

Boiling points, H2SO4

Viscosity, H2SO4

Heat of dilution, H2SO4 with H2O liquid

Enthalpy SO3-H2O system

Typical dilutions H2SO4 with water both
at 80°F

Unlined steel corrosion by sulfuric acid

  • Keep the water flowing through the straight run of the mixing tee, and bring the sulfuric acid in the side outlet.
  • Provide check valves in both the acid and water lines to prevent backflow and unwanted mixing, which could occur if there were a flow interruption of either stream.
Mixing tees with PTFE nozzles specially designed to disperse the acid uniformly into the process are available in 1" through 8" sizes.

To add a margin of safety, it is recommended that PTFE-lined pipe be utilized in the immediate mixing area, upstream of both check valves and approximately 20' downstream of the mixing area.

  • Size the piping to ensure turbulent flow for good mixing and a minimum of hot spots.
  • Provide the point for acid dilution as close as possible to the point at which the diluted acid will be used or stored. This will minimize the amount of piping required to handle the hot acid.

Using Sulfuric Acid with Polypropylene-lined Pipe

Resistoflex does not recommend the use of polypropylene-lined pipe, fitting and valves in the 93% - 98% sulfuric acid concentration range. In this application, polypropylene is susceptible to a liquid oxidative degradation mechanism which leads to dehydrogenation, charring of the polypropylene liner and brittle failure of the plastic liner.

The overall failure mechanism of polypropylene-lined components in concentrated sulfuric acid applications is comprised of three individual steps:

  • liquid oxidative degradation reaction,
  • the diffusion of sulfuric acid into the polymeric matrix, and
  • the brittle response of the polypropylene liner
Each affect the service life of the polypropylene-lined components. For example, the liquid oxidative degradation reaction is significantly affected by temperature, pressure, sulfuric acid concentration and secondary chemical concentration (e.g. free SO2 and SO3). The diffusion of sulfuric acid is influenced by temperature, pressure, sulfuric acid concentration, as well as density and crystallinity of the polypropylene liner. Finally, the brittle response of the polypropylene liner is affected by internal, mechanical and thermal stresses.

This limitation does not mean Resistoflex's polypropylene-lined components cannot be used on more dilute sulfuric acid concentrations. In fact, in concentrations less than 93%, polypropylene is quite an acceptable choice.

Liquid Oxidation of Polypropylene by Concentrated Sulfuric Acid

Polypropylene (PP)-lined pipe and fittings are known to stress crack in concentrated sulfuric acid (93% - 98%) applications.

An extensive literature search and analytical research have indicated that polypropylene is susceptible to liquid oxidative degradation by concentrated sulfuric acid. Polypropylene, like polyethylene, is a member of the polyolefin family. Polyolefins are prone to degradation by thermo- and photo-oxidation as well as gamma-radiation and liquid oxidative degradation by strong oxidizing agent such as concentrated sulfuric acid and nitric acid.

The following chain reaction mechanism is followed in the liquid oxidative degradation of polypropylene by sulfuric acid.

This mechanism leads to dehydrogenation and ultimately to charring of the polypropylene liner. Upon charring, the physical evidence of liquid oxidative degradation, the plastic fails in a brittle manner. Crack initiation in the plastic liner can be accelerated by one or a combination of the following: mechanical, thermal, and internal stresses.

The liquid oxidative degradation mechanism of polypropylene can be influenced by many factors which in turn affect the service life of polypropylene-lined pipe and fittings in sulfuric acid. Service life of PP-lined pipe and fittings in concentrated sulfuric acid has varied from 6 months to as much as 20 years. Laboratory chemical resistance tests of PP-lined pipe and fittings in 96% sulfuric acid have produced failures in as little as 2 weeks. The following table contains a list of factors that can influence the rate of liquid oxidative degradation.

Parameter Change in Parameter Effect on Liquid Oxidative Degradation of Polypropylene
Temperature + +
Pressure + +
Sulfuric Acid Concentration + +
Secondary Chemical Concentration (e.g. free SO2) + +
Internal Stresses + +
Mechanical Stresses + +
Surface Area to Volume Ratio + +
Liner Density + -
Crystallinity + -
Molecular Weight + -
Residual Antioxidant Concentration + -

Chemical resistance guides published by polypropylene resin manufacturers state that polypropylene is appreciable affected by 98% sulfuric acid at room temperature. This guide also states that, if the plastic is under stress, failure can occur with concentrated sulfuric acid at temperatures lower than room temperature. For further details, please see the Chemical Resistance Guide.

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