Incineracion De Residuos De Clorohidrocarburos

The use of carbon fiber reinforced DIABON( graphite tubes in shell and tube phosphoric acid evaporators



Clearwater, Florida
June 2009


By Loïc BERNARD
Core Products Manager
SGL Group ProcessTechnology

Abstract:

For numerous decades, resin impregnated graphite has been the material of choice for the construction of phosphoric acid evaporators. The material has an outstanding resistance to every type of green phosphoric acid produced in the world and this independent of their fluoride and chloride content. When totally graphitised, the material also posts an outstanding thermalconductivity of approximately 80 W/m.K.. Which makes it a suitable material for the manufacturing of heat exchangers. Unfortunately, the material also has a reputation in the phosphoric acid industry for its sensitivity to mechanical and thermal shocks as well as its limited resistance to erosion.

SGL Group has developed in the early 1990’s a carbon fiber reinforced graphite tube thatsubstantially alleviates the previously described inconveniences while maintaining the performance characteristics of the material and bringing operational reliability to the green phosphoric acid evaporation process. The first installed evaporators with carbon fiber reinforcement have been now in operation for more than 20 years. This gives us a clear picture of the real benefits of this technology andhelps to suggest additional measures to progress further towards operational excellence.

The results of the tests originally carried out in our laboratories when we developed the carbon fiber reinforced tubes have been confirmed by the experiences accumulated by all of our customers around the world. It shows a significant improvement of the tubes mechanical properties resulting in a substantialreduction in the number of broken tubes. Commonly, a reduction by a factor of 20 times has been commonly observed in the number of tubes broken in operation, thus resulting in an extension of the equipment lifetime.

These years of operation also suggests that the use of a chemical cleaning method instead of water blasting improves even further the equipment reliability. In addition, it suggeststhat the proper design of steam and condensate lines is another key to operational reliability and equipment lifetime.


Process and Process Conditions:


The different wet phosphoric acid processes produce green acids with concentrations between 28 and 30 % P2O5 for the conventional dihydrates (DH) process either with a single or multiple reaction tanks, between 30 and 32 % P2O5 for thehemihydrates recrystallization (HRC), between 32 and 36 % P2O5 for the dihemihydrates (DHHH), between 40 and 42 % P2O5 for the hemihydrates (HH) and between 40 and 54 % P2O5 for the hemidihydrates (HDH).

For many technical (precipitation of impurities, corrosion resistance, temperature) and economical (evaporation is expensive but transporting water is as well) reasons, the phosphoriccommercial grade has been set at 54%. This means, that depending on the production process, the acid coming out from the reactor tanks must be concentrated in 1 to 3 evaporation stages.

Through the years, many types of processes have been developed for concentrating phosphoric acid. Originally, submerged combustion concentrators where gas or fuel was burned directly in a pool of phosphoric acid andcaused the concentration of the acid. These units were extremely pollutive and paved the way to vacuum concentrators. Natural circulation evaporators were used for a while until the actual design was adopted, i.e. forced circulation evaporators. The typical evaporation stages consist of an axial pump, a heat exchanger and a flash tank. The largest axial pumps can convey up to 12,000 m3/h (50,000...