Chemical Engineering June 2020 - 42

Table 2. compoSiTion of The SpenT
acid for The example
Component
H2SO4
HNO3
H2O
Organics
Content
65 wt.%
10 wt.%
22 wt.%
3 wt.%
70 wt.%. absorption
of nox
to produce nitric
acid involves a
gas-phase oxidation
of nitrogen
monoxide
and absorption
of nitrogen dioxide
in water [see equations (3) and (4)]. the gas-phase
oxidation as well as the dissolution of nitrogen dioxide
in aqueous solution is favored by increased gas pressure.
the absorption is performed in stainless-steel
tray columns.
2no + o2 → 2no2
2no2 + H2o → Hno3 + no
(3)
(4)
evaporation of sulfuric acid above 90 wt.% produces
a so3-rich gas. the absorption of this so3 increases the
concentration of sulfuric acid. the absorption does not
necessarily require an increased gas pressure and can
be performed in the same pressure system as the initial
evaporation step.
Design criteria
the selection of the process modules is determined by
the composition of the sa and the required concentration
and quality of the product acids. for the removal of
contaminants, modules a to C are selected depending
on the type of contaminant. to determine the adequate
design parameters for these process modules, depending
on the sa to be treated, laboratory tests or even pilot
plant tests may be required prior to a commercial design.
if a separation of a sa consisting of nitric and sulfuric
acid is necessary, the process module d is used. depending
on the target concentration of the sulfuric acid
product, the evaporation modules e or f are applied or
combined. With the aim of achieving high energy efficiency
within the system, vacuum evaporation (module
f) is used when very low (<15%) concentrated acid must
be recycled or high sulfuric acid concentrations shall be
achieved. the absorption modules are used for internal
recovery of nox (module g) or so3 (module H) whenever
large quantities of these gases are produced. an
Module G:
pressure
absorption
HNO3
Module A:
stripping
SA
1,000 kg/h
Module B:
thermal
oxidation
Module D:
rectification
Module F:
vacuum
Figure 5. This block flow diagram is used for the
example described in the text. The composition of the
spent acid (SA) is shown in Table 2
42
evaporation
98.5 wt.%
125 kg/h
Module H:
pressureless
absorption
H2SO4
96 wt.%
675 kg/h
efficient energy recovery is achieved by using the energy
provided in the previous process step (for example,
thermal oxidation) in the subsequent steps (for example,
evaporation). the sequential design of process steps
also protects sensitive downstream equipment by initially
removing contaminants.
Example: Recycling of nitration acid
as an example, a recycling process for the treatment of
nitration sa is shown in the following. it combines various
process modules to produce concentrated nitric
acid of 98.5 wt.% and sulfuric acid of 96 wt.% from a
sa. the sa originates from nitroglycerine production
with the composition shown in table 2. the process
design shown in figure 5 considers a mass flowrate of
1,000 kg/h sa. all product acids are exported with 40°C.
the illustrated exemplary process comprises the decomposition
of organic contaminants by thermal oxidation
and a subsequent rectification to separate the acid
mixture. the nitric acid is separated as distillate and
cleaned from volatile nox by stripping to produce a clear
acid. the nox are recovered as nitric acid in a pressure
absorption. the sulfuric acid from the rectification column
bottom is concentrated to 96% by vacuum evaporation
while emerging so3 is recovered as sulfuric acid.
Heating, evaporation, condensation and cooling in the
thermal recycling process requires heating and cooling
energy. this exemplary process requires approximately
460 kW heat and 480 kW cooling energy to produce
125 kg/h of concentrated nitric acid and 675 kg/h of sulfuric
acid. Besides the positive environmental effect of
recycling, the feasibility of recycling sa rather than disposing
it and buying fresh nitric and sulfuric acids from
the market has to be evaluated by plant operators who
are considering a financial investment into a sa recycling
plant. due to the high costs for sa disposal and fresh
acid procurement, recycling can significantly reduce the
operating expenses (opex) for any production process
where great quantities of sa arise.
in order to determine the financial viability of such a
capital investment project, the plant operators will have
to perform a comprehensive financial assessment study
that considers the initial capital expenses (capex) and location-specific
opex including energy cost, disposal cost
for hazardous waste, product acid pricing.
Final remarks
in order to recycle spent acids from
various applications, a flexible design
approach, applying process modules,
enables operators to deal with
spent acids containing different contaminants.
the overall process design
is combining independent process
modules to an overall recycling
process that can recycle different sa
with varying acid concentrations and
containing organic or inorganic contaminants
to achieve different acid
concentrations and purities dependChemiCal
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