Instructional Manual For Clarification Of Startup In Source Categories Affected By New Source Performance Standards - U.s. Environmental Protection Agency - 1979 Page 101
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146 SUPERPHOSPHORIC ACID PLANTS - SUBPART U
§60.2l0 - 60.214
Introduction
Superphosphoric acid plants regulated under this part are defined as any
facility which concentrates wet-process phosphoric acid to 66 percent or greater
phosphorous pentoxide (P20S) content by weight for eventual consumption as a
fertilizer.
Affected facilities include any combination of evaporators, hot-
wells, acid sumps, and cooling tanks.
Total fluoride emissions are limited to
5 g per metric ton (0.01 lb/ton) of equivalent P20S feed, defined as the
quantity of phosphorous pentoxide fed to the process.
Sources constructed,
reconstructed or modified after October 22, 1974, are subject to the
regulation.
Process Description
Superphosphoric acid is produced by concentrating 54 percent PzOs phos-
phoric acid to about 70 percent
(±
2 percent) PzOS'
Two commercial processes
used to accomplish this are vacuum evaporation and submerged combustion, al-
though the latter process is virtually outdated.
In the vacuum evaporation
process, depicted in Figure 19, clarified 54 percent PzOS acid is continuously
fed to a vacuum evaporator from which hot gases containing water vapor and
fluorides are condensed in the water-cooled barometric condenser.
Condenser
water flows to the hotwell prior to draining to the gypsum pond.
Concentrated
acid is drawn from the evaporator to cooling tanks and then to storage.
In the submerged combustion process, Figure 20, hot gases are forced
below the surface of the 54 percent P20S phosphoric acid in a submerged com-
bustion evaporator.
Water vapor, fluorides, and acid mist are driven from
solution and concentrated acid is drawn off as product.
Pre-Startup Operations
The major piece of equipment requiring a thorough mechanical checkout is
the evaporator.
In the past, falling-film evaporators have been used but have
b~en
replaced more recently with forced-circulation units.
A disadvantage of
the falling-film type was the inability to uniformly distribute the liquid
a8 a film inside the exchanger tubes.
The evaporator is typically a shell and tube exchanger with high pressure
steam input, a high-volume, low-head, recirculation pump, and is operated at
about 6.75 to 20.3 kPa (2 to 6 in. Hg) absolute pressure.
Initial checking of
the evaporator assembly is carried out by the contractor while the plant will
89
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