Chemical Engineering June 2020 - 40

a)
Contaminants
Spent acid
b)
only module B
Oxidizer
Spent acid
Air/steam
Cleaned acid
only
module C
Figure 3. Shown here are flow diagrams a) for module A (stripping) and b) for the modules B and C (thermal
oxidation and precipitation/filtration). The oxidizer is only added in module B and the filtration only
used in module C
action vessels) made of corrosionand
temperature-resistant materials
these unit operations form process
modules that can be combined to
treat various sa. in general, each
process module serves one of the
three above-described steps in the
sa recycling process as is further
described below. table 1 shows a
summary of the process modules.
Step 1: Removal of contaminants.
the contaminants or impurities
found in the sa can be classified
as volatile or non-volatile. modules
for the removal of these contaminants
are shown in figure 3. Volatile
compounds can be removed from
a high-boiling sa by stripping with
air or steam (module a). in general,
the vapor pressure of many organic
compounds (for example, nitrobenzene)
and some inorganic compounds
(such as hydrogen fluoride
or oxides of nitrogen) increases with
temperature. therefore steam stripping
promotes the separation. for
an optimal gas-liquid contact, stripping
is performed in packed bed
columns. these columns are made
of either glass-/polymer-lined steel,
a)
Nitric acid
Mixed acid
Diluted acid
NOx/SO3
Sulfuric acid
Concentrated acid
only
module G
Concentrated acid
fiber-reinforced polymer or stainless
steel. By stripping these impurities
from the sa, it is possible to recover
them in a condensation or absorption
step downstream of the stripping
unit. However, in some cases,
recovery is not possible due to a severe
safety hazard being posed by
some organic nitrates.
non-volatile or hazardous volatile
organic compounds can be destroyed
by thermal oxidation (module
B). the oxidation must destroy all
organics non-selectively to achieve
a purified acid and protect downstream
equipment from explosive
compounds. in order to achieve a
complete oxidation, the sa must be
heated up to temperatures between
120 and 200°C. for diluted acids
whose boiling point is lower than
the required decomposition temperature,
sulfuric acid can be added
to achieve an oxidation in the liquid
phase, resulting in smaller equipment
sizes. nitric or mixed acids already
contain nitric acid as oxidizing agent,
while for sa without nitric acid, an
oxidizing agent consisting, of hydrogen
peroxide or nitric acid, for exb)
only
module
F
Water
Diluted acid
c)
Off-gas
Cleaned acid
Off-gas
ample, must be added. an example
of a reaction of an organic impurity
(benzene) with hydrogen peroxide or
nitric acid is shown in equations (1)
and (2). figure 2 depicts a sulfonation
sa before and after the thermal
oxidation step.
C6H6 + 15H2o2 → 6Co2 + 18H2o
(1)
2C6H6 + 30Hno3 →
12Co2 + 21H2o + 15no + 15no2
(2)
precipitation and subsequent filtration
removes non-volatile inorganic
compounds (module C). the precipitation
/ crystallization is facilitated
by increasing the concentration of
the inorganic impurities beyond the
solubility, which depends on the
temperature and acid concentration.
this can be achieved by evaporation
of excess water and/or cooling of
a hot sa to decrease the solubility.
a higher acid concentration usually
also decreases the solubility of inorganic
compounds and therefore has
a positive influence on remaining impurities
in the system.
Step 2: Separation of acid mixtures.
the separation of acid mixtures
is done by rectification, as
shown in module d (figure 4a).
the higher boiling sulfuric acid is
discharged at the column bottom
and the nitric acid as distillate
from the column overhead. due to
the hygroscopic nature of sulfuric
acid, the water from the feed sa
is discharged with the sulfuric acid
whereby high concentrations of nitric
acid can be achieved. the highly
Figure 4. Shown here are the flow diagrams for various modules. Figure 4a is module D (rectification), in which the nitric acid is separated from the acid mixture
as distillate and the sulfuric acid and water as bottoms. Figure 4b is modules E and F (atmospheric and vacuum evaporation) and Figure 4c is for modules G
and H (pressure and pressureless absorption). Vacuum pumps and compressors are only used for modules F and G
40
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