- 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.
