Structured Packing in Nitric Acid Concentration: Azeotrope, Extractive Distillation and Corrosion Control
Nitric acid concentration is a useful example of a separation where adding more theoretical stages eventually stops solving the problem.
Weak or medium-strength nitric acid can be concentrated by ordinary rectification until the nitric acid-water system approaches its azeotropic composition. At atmospheric pressure, this occurs at roughly 68 wt% HNO₃.
Once the column reaches that region, ordinary binary distillation cannot simply continue producing 98–99% nitric acid by adding more trays or more structured packing.
Industrial high-concentration nitric acid plants therefore change the thermodynamics of the system, commonly by adding a strongly hygroscopic dehydrating agent such as sulfuric acid. Magnesium nitrate processes have also been used.
Structured packing may provide the vapor-liquid contacting required inside parts of this concentration system, but the packing does not remove the azeotrope.
Its role begins only after the process has created a workable separation route.
Why Ordinary Rectification Stops Near 68% Nitric Acid
Ammonia oxidation plants commonly produce nitric acid in approximately the 50–68 wt% range.
For many fertilizer applications, this concentration is already sufficient.
Other industries require much stronger acid, including certain:
- nitration processes
- specialty chemical processes
- energetic-material processes
- polyurethane intermediate production
The difficulty is the HNO₃-water vapor-liquid equilibrium.
At approximately 68 wt% nitric acid, the liquid and vapor reach azeotropic behavior at atmospheric pressure. De Dietrich identifies this azeotrope at about 68 wt% HNO₃ and approximately 122°C.
That creates a hard boundary for ordinary rectification.
A tower can approach the azeotrope.
It cannot simply cross it because someone installs another five meters of 250Y packing.
That distinction is essential when reviewing a nitric-acid RFQ.
If the customer asks for:
60% → 67% HNO₃
the project may primarily be a pre-concentration duty.
If the target is:
60% → 98.5–99.8% HNO₃
a different process mechanism is required.
Sulfuric Acid Changes the Separation
One established solution is extractive rectification using concentrated sulfuric acid.
Sulfuric acid strongly associates with water and changes the vapor-liquid equilibrium of the HNO₃-H₂O system.
De Dietrich states that addition of concentrated sulfuric acid removes the nitric-acid/water azeotropic limitation and allows nitric acid to be concentrated up to approximately 99.8 wt%.
Conceptually, the process becomes:
weak nitric acid + concentrated sulfuric acid → extractive concentration → concentrated nitric acid overhead + diluted sulfuric acid bottoms
The diluted sulfuric acid must then be reconcentrated and returned to the process.
So the high-concentration nitric acid column is not operating independently.
It belongs to an acid-recycle system involving:
- nitric acid feed
- sulfuric acid dehydrating agent
- concentration column
- sulfuric acid regeneration
- product condensation
- NOx treatment
Structured packing therefore has to work with the complete flowsheet.
Magnesium Nitrate Provides Another Route
Magnesium nitrate has also been used as a dehydrating agent.
Classic nitric-acid concentration processes mix weak nitric acid with a concentrated magnesium nitrate solution. The salt suppresses water volatility sufficiently for concentrated nitric acid to be recovered, after which the diluted magnesium nitrate solution is reconcentrated and recycled.
One process describes roughly 60% nitric acid being contacted with approximately 72% magnesium nitrate solution, ultimately producing concentrated nitric acid while the Mg(NO₃)₂ solution is regenerated through vacuum evaporation.
This matters to the packing supplier because:
HNO₃ + H₂O + H₂SO₄
and
HNO₃ + H₂O + Mg(NO₃)₂
are not the same hydraulic or corrosion service.
A quotation saying only:
“Packing for 98% nitric acid production”
is not enough.
The concentration technology needs to be identified first.
Structured Packing Can Appear in the Acid-Recovery Sections
There is direct industrial evidence for structured packing in nitric-acid concentration-related service.
A Chinese industrial process describes a rectification column approximately 1200 mm in diameter and 9000 mm long filled with stainless-steel structured packing. The tower operated under vacuum to separate dilute nitric acid from acidic water generated in a magnesium-nitrate concentration process, recovering nitric acid in the bottom while producing very low-acid condensate overhead.
This example is useful because it shows that “nitric acid concentration plant” may contain more than the main high-strength acid column.
Structured packing opportunities may occur in:
- weak-acid pre-concentration
- acid-water recovery
- dehydrating-agent regeneration
- rectification
- bleaching or stripping sections
Each one requires a separate corrosion and hydraulic review.
DAIER should therefore ask for the equipment tag and stream composition, rather than assuming every tower in an NAC unit contains the same packing.
Corrosion Can Override the Hydraulic Optimum
This is one of the most important differences between nitric-acid concentration and ordinary solvent distillation.
Hot concentrated nitric acid is highly corrosive, and adding sulfuric acid can create an even more demanding material environment.
De Dietrich specifically notes that nitric acid above the azeotropic concentration is corrosive to many stainless steels and uses highly corrosion-resistant construction such as borosilicate glass and glass-lined steel in its concentration systems. Packing and other column internals are part of that corrosion-resistant equipment package.
That means a supplier should never reason:
“It is structured packing, so use SS316L.”
The correct material may instead depend on:
- acid concentration
- sulfuric acid concentration
- operating temperature
- NOx content
- water content
- process section
- plant corrosion specification
Depending on the duty, the project may use ceramic, glass, glass-lined equipment, specialty metals or another approved material system.
The process engineer's corrosion specification has priority over the packing catalogue.
Ceramic Packing Can Become Relevant for a Different Reason
Metal structured packing is attractive when efficiency, mechanical strength and thin-sheet geometry are priorities.
But very aggressive mineral-acid systems often create strong interest in non-metallic materials.
Ceramic structured packing can offer excellent chemical resistance in appropriate acid environments.
The trade-off is different.
Ceramic internals require attention to:
- brittleness
- segment handling
- support design
- thermal shock
- installation loading
- mechanical impact during maintenance
Therefore, a ceramic structured packing proposal should not simply copy the dimensions of a stainless-steel packing block.
The support system and installation method need to reflect the material.
For DAIER, this is especially relevant because acid concentration projects may become combined inquiries for:
structured packing + support grid + distributor + collector
rather than packing alone.
Pressure Drop Still Matters Even Without Deep Vacuum
Nitric-acid concentration is often dominated by thermodynamics and corrosion, but hydraulic pressure drop remains important.
A tall concentration section may require many effective stages.
Every unnecessary pressure loss changes the pressure and boiling-temperature profile through the tower.
In recovery or regeneration sections operated under vacuum, this becomes even more important.
The industrial magnesium-nitrate acid-water recovery example used vacuum operation around −0.09 to −0.07 MPa gauge with structured packing.
Under those conditions, a low-resistance packing bed helps preserve the intended vacuum through the full separation section.
But the lowest-pressure-drop packing is not automatically best.
If the geometry becomes too open, stage efficiency may fall and more bed height may be required.
The design still needs a balance between:
efficiency + capacity + pressure drop + corrosion resistance.
NOx Means Product Purity Is More Than HNO₃ Concentration
Producing 98–99% nitric acid does not automatically mean the product is finished.
During high-concentration nitric-acid processing, nitrogen oxides can form or dissolve into the acid.
This can give the concentrated acid an undesirable color and affect product specification.
Industrial concentration plants therefore commonly include a bleaching or stripping step after concentration.
De Dietrich describes air stripping of concentrated acid to remove dissolved NOx, with the separated NOx subsequently recovered rather than simply released.
This creates another potential packed-column duty.
The nitric-acid concentration train may therefore contain:
- pre-concentration,
- extractive concentration,
- dehydrating-agent regeneration,
- concentrated-acid bleaching.
These should not be collapsed into one “nitric acid tower.”
Distributor Material Is Just as Important as Packing Material
It would make little sense to specify highly corrosion-resistant packing while leaving the liquid distributor in an unsuitable material.
The distributor sees some of the most demanding liquid exposure in the column and must maintain uniform flow across the bed.
Material compatibility therefore needs to cover:
- distributor
- packing
- support grid
- hold-down arrangement
- collectors
- fasteners
- feed pipes
At the same time, distributor geometry needs to handle the actual liquid.
An extractive acid column can carry a much heavier liquid load than a conventional nitric-acid rectifier because the dehydrating agent contributes substantial circulation.
This is especially relevant with magnesium-nitrate processes, where the dehydrating solution flow can be several times the weak-acid feed. Classic process descriptions show large magnesium-nitrate-to-acid circulation ratios.
So hydraulic design cannot use nitric-acid feed rate alone.
Salt-Based Processes Add Crystallization Questions
A magnesium-nitrate process introduces another concern that a sulfuric-acid process does not have in exactly the same form.
The dehydrating agent is a concentrated salt solution.
If temperature or concentration moves outside the intended operating range, crystallization behavior needs to be considered.
For structured packing and distributors, any salt deposition can progressively affect:
- liquid passages
- distributor openings
- surface wetting
- pressure drop
This does not mean structured packing is unsuitable.
It means the plant's concentration and temperature window must remain within the licensed process conditions.
For a replacement project showing deposits, DAIER should ask whether those deposits are:
- magnesium salts
- corrosion products
- process contamination
before changing packing geometry.
Different deposits imply different solutions.
What DAIER Needs for a Nitric Acid Concentration RFQ
The first question should be:
Which section of the nitric acid concentration plant?
Then the project should define at least:
- weak nitric acid concentration
- required product HNO₃ concentration
- dehydrating agent
- sulfuric acid or magnesium nitrate concentration
- feed rate
- circulating dehydrating-agent rate
- operating pressure
- temperature profile
- vapor and liquid loads
- tower inside diameter
- available bed height
- target acid recovery
- allowable pressure drop
- approved packing material
- distributor material
- support-grid material
- NOx handling requirements
- existing internals for retrofit projects
For replacement packing, the existing material specification is particularly important.
An apparently identical geometry made from the wrong material is not an equivalent replacement in concentrated nitric-acid service.
The Azeotrope Defines the Process Before Packing Selection Begins
Nitric acid concentration shows clearly where structured packing engineering begins and where it ends.
Packing can improve:
- vapor-liquid contacting
- effective stage density
- pressure drop
- hydraulic capacity
But it cannot change the fundamental HNO₃-water azeotrope by itself.
To move from ordinary 55–68% nitric acid toward 98–99.8%, the process first needs a thermodynamic solution such as extractive rectification with sulfuric acid or another proven concentration technology.
Only after that should the packing question be asked:
Which corrosion-resistant contacting system can deliver the required separation at the actual acid, dehydrating-agent and hydraulic loads?
That is the correct role of structured packing in high-concentration nitric acid production.