Structured Packing in Nitric Acid Absorption Towers: NOx Absorption, Cooling and Liquid Distribution

Structured packing can be used in nitric acid absorption service, but a nitric acid absorber should not be treated like an ordinary gas scrubber.

The tower is handling a reactive NOx system in which gas-phase oxidation, absorption into water or dilute nitric acid, heat release and acid concentration all interact. Good mass-transfer area is important, but so are temperature control, oxygen availability, liquid distribution and corrosion resistance.

For that reason, structured packing can be attractive in selected nitric acid absorption sections, especially where high gas-liquid contact efficiency and low hydraulic resistance are useful. It does not automatically replace the conventional tray arrangement used in many nitric acid plants.

The real design question is:

Can the packed section improve NOx absorption without losing the cooling and reaction control required by the nitric acid process?

Nitric Acid Absorption Is a Reactive System

In nitric acid production, ammonia is first oxidized to form nitrogen oxides.

The process gas then passes through cooling and oxidation steps before entering the absorber, where nitrogen oxides are contacted with water or dilute nitric acid.

This is not simple physical absorption.

Several chemical and mass-transfer steps occur together. Nitric oxide, nitrogen dioxide and dinitrogen tetroxide can participate in oxidation and absorption reactions, and the liquid becomes progressively richer in nitric acid as it travels down the tower.

The gas composition also changes from one elevation to another.

This means the duty of the lower absorber section can be very different from that of the upper polishing section.

A single “NOx flow rate” does not fully describe the tower.

Temperature Directly Affects Absorption Performance

NOx absorption into aqueous nitric acid is favored by suitable temperature conditions.

But the absorption reactions release heat.

If the liquid becomes too warm, absorption performance can deteriorate and more NOx may remain in the tail gas.

This is one reason conventional nitric acid absorption towers often include extensive cooling arrangements.

Structured packing provides mass-transfer surface, but it does not remove the heat of reaction by itself.

If a plant considers replacing trays or another contacting device with packing, the engineer must still answer:

• Where will the reaction heat go?
• How will liquid temperature be controlled?
• Does the existing cooling system remain adequate?
• Will the temperature profile change after the retrofit?

A lower-pressure-drop internal does not compensate for insufficient cooling.

Why Structured Packing Can Be Attractive

A structured packing bed provides a large, ordered contact surface while maintaining open passages for gas flow.

For NOx absorption, this can offer several potential advantages:

• efficient gas-liquid contacting
• relatively low gas-side pressure loss
• compact contacting volume
• predictable liquid distribution when the distributor is well designed

Recent nitric-acid absorber developments have even investigated structured packing in specific sections of production absorbers to improve residual NOx capture. One recently issued design describes structured packing combined with a dedicated liquid distributor in a nitric acid absorption tower rather than simply treating the packing as a drop-in replacement for ordinary internals.

That distinction matters.

The benefit comes from packing + liquid distribution + process arrangement, not the packing block alone.

The Top and Bottom of the Absorber Have Different Jobs

The lower part of the absorber normally sees a richer NOx gas.

As the gas rises and absorption proceeds, the remaining NOx concentration falls.

Near the upper part of the column, the process increasingly becomes a polishing duty: relatively small remaining quantities of nitrogen oxides must be removed before the tail gas leaves the absorber.

This changing duty can affect whether structured packing is valuable.

A high-efficiency packed section may be particularly interesting where:

• residual NOx removal is important
• gas pressure-drop allowance is limited
• liquid loading is suitable
• cooling can be managed

But it does not follow that every tray in the complete nitric acid absorber should be removed and replaced by packing.

Different elevations may need different contacting and heat-removal strategies.

Liquid Distribution Is Critical

Structured packing only works efficiently when the liquid reaches the full bed cross-section.

This becomes especially important in nitric acid service because the liquid is not merely a wetting medium. It is also part of the reactive absorption system.

If one region receives too little liquid:

• effective contact area falls
• local NOx absorption can deteriorate
• gas can pass through underused channels

If another region receives too much liquid:

• local hydraulic loading increases
• the acid concentration and temperature profile may become uneven
• available gas passage can be reduced

The liquid distributor therefore deserves as much attention as the nominal packing surface area.

For a large nitric acid absorber, “we need 250Y structured packing” is not a complete internals specification.

The engineer also needs to know how the acid will be distributed over it.

Oxygen Availability Is Part of the Chemistry

One subtle difference between nitric acid absorption and many conventional scrubbers is that absorption performance depends partly on the oxidation state of the nitrogen oxides.

NO is much less readily absorbed directly than NO₂/N₂O₄ under typical absorber conditions.

Oxidation of NO toward more absorbable nitrogen oxides therefore matters to overall performance.

That reaction requires both oxygen and sufficient process conditions for oxidation to occur.

A packing bed may improve gas-liquid contacting, but it cannot overcome an unfavorable upstream gas composition simply by providing more surface area.

If a nitric acid plant has unexpectedly high NOx emissions, the diagnosis should therefore consider:

• oxidation conditions
• gas composition
• absorber temperature
• pressure
• acid concentration
• liquid distribution
• gas-liquid contacting

before concluding that the packing area is insufficient.

Pressure Can Change the Value of the Packing

Nitric acid absorption is often operated above atmospheric pressure because increased pressure can improve the overall absorption environment and reduce gas volume.

This changes the role of packing selection.

At higher pressure, gas density increases and volumetric gas flow can be lower for the same mass flow. At the same time, the chemistry and absorption equilibrium differ from a low-pressure scrubber.

The structured packing therefore needs to be evaluated at the actual absorber pressure, not at standard conditions.

For a preliminary review, DAIER would need the real operating gas volume or sufficient composition, pressure and temperature data to determine it.

Using Nm³/h directly as if it were the actual tower gas volume can give the wrong hydraulic picture.

Material Selection Needs More Than the Word “Nitric Acid”

Nitric acid is a strongly oxidizing medium.

That does not mean one universal stainless-steel grade should be recommended for every absorber.

Material compatibility depends on the combined conditions, including:

• acid concentration
• operating temperature
• NOx environment
• water content
• impurities
• fabrication requirements

DAIER’s own engineering selection logic treats nitric acid separately as oxidizing service and requires the oxidizer identity, concentration, impurities and temperature to be reviewed rather than applying a generic “acid-resistant” material rule.

This is important because a material that performs well in one nitric acid concentration range may not be the automatic choice under another combination of temperature and contaminants.

The packing material, distributor and support internals should therefore be reviewed as one corrosion system.

A Retrofit Should Start With the Tail-Gas Problem

If an existing nitric acid plant is considering structured packing because NOx emissions are too high, the first task is not choosing a packing model.

The plant should identify why absorption performance is inadequate.

Possible causes include:

• insufficient contacting
• poor liquid distribution
• high liquid temperature
• weak oxidation upstream
• excessive production rate
• inadequate cooling
• incorrect acid circulation
• hydraulic limitation
• damaged or fouled internals

Only some of these problems are solved by installing new packing.

If the real limitation is cooling, changing the contacting device may provide little improvement.

If the upper tower section is mass-transfer limited while the rest of the absorber performs well, a targeted packed-section modification may be more rational than rebuilding the complete absorber.

What Should Be Included in a Nitric Acid Absorber RFQ?

For an initial structured packing review, useful project information includes:

• nitric acid production capacity
• absorber inside diameter
• operating pressure
• gas flow at actual conditions
• inlet NOx composition
• oxygen concentration, if available
• liquid flow
• acid concentration by relevant section
• operating temperature
• cooling arrangement
• required tail-gas NOx level
• existing contacting internals
• available packed height
• material specification
• known corrosion or fouling history

For retrofit work, operating data before and after any recent capacity increase are particularly valuable.

The supplier should understand whether the objective is:

• increased nitric acid production
• reduced tail-gas NOx
• lower pressure drop
• replacement of damaged internals
• tower debottlenecking

These objectives do not necessarily lead to the same packing design.

Structured Packing Is One Part of the NOx Absorption System

A nitric acid absorber cannot be optimized by maximizing packing surface area alone.

The complete process depends on:

NOx oxidation + gas-liquid contact + heat removal + acid circulation + pressure + corrosion control.

Structured packing can contribute strongly to the contacting part of that system.

Its low hydraulic resistance and ordered mass-transfer surface can be useful, particularly in sections where efficient NOx removal is required without unnecessary pressure loss.

But the correct selection begins with the nitric acid process itself.

For this service, the best question is not:

“Which structured packing has the highest efficiency?”

It is:

“Which part of the absorber is limiting NOx capture, and can a packed section improve that duty while preserving the required temperature and reaction conditions?”