Maleic Anhydride Recovery Absorbers: Why Structured Packing Belongs in the Cooling Sections, Not Everywhere

A maleic anhydride recovery absorber is a good example of why structured packing should not be specified for an entire column simply because it offers low pressure drop and high mass-transfer area.

In one published industrial recovery configuration, structured packing or metal grid packing is preferred in two lower gas-cooling and absorption sections, where large recirculating solvent flows remove both sensible heat and heat of absorption.

Yet farther up the same column, the main and final maleic anhydride absorption sections use specially designed trays instead.

The reason is not that one internal is universally more efficient.

The hydraulic duty changes sharply from one section to another.

The cooling zones have relatively high liquid circulation and benefit from open, low-pressure-drop internals with short liquid residence time. The upper absorber has much lower liquid loading and a very different gas-to-liquid ratio, making trays easier to control and distribute reliably.

Maleic anhydride chemistry adds another constraint: the column must remain inside a narrow temperature window. If it becomes too hot, absorption deteriorates and maleic-acid-to-fumaric-acid conversion becomes more troublesome. If it becomes too cold, water condensation and fumaric-acid precipitation can create solid deposits.

The correct question is therefore not:

Which structured packing should be installed in the maleic anhydride absorber?

It is:

Which sections actually benefit from structured packing, and which sections need a different internal because the hydraulic and chemical conditions have changed?

One Absorber Can Contain Several Completely Different Duties

Maleic anhydride can be produced by catalytic oxidation, commonly from hydrocarbon feedstocks such as n-butane.

The reactor effluent contains maleic anhydride together with hot process gas, water vapor and reaction byproducts.

Before the maleic anhydride can be purified, it has to be recovered from this gas stream.

One established process route uses an organic absorbent such as a dialkyl phthalate ester. Dibutyl phthalate is one disclosed example. The rich solvent leaving the absorber can contain approximately 8–24 wt% maleic anhydride in the cited process.

But calling the equipment simply a “maleic anhydride absorber” hides what is actually happening inside it.

The published configuration contains several functional zones:

hot reaction-gas feed zone

↓

first gas-cooling / absorption section

↓

second gas-cooling / absorption section

↓

main absorption section

↓

final polishing absorption section

with an additional stripping function below the feed in the disclosed integrated arrangement.

Each section has a different:

• gas temperature;
• maleic anhydride concentration;
• liquid circulation rate;
• liquid-to-gas ratio;
• heat load;
• solids risk.

That is why one internal technology is not automatically optimum from bottom to top.

The First Packed Section Is Doing Heat Transfer as Well as Absorption

The hot reactor gas entering the absorber is not yet at the ideal temperature for final absorption.

In the published process, the first cooling section reduces reaction-gas temperature from roughly 130–200°C to around 70–85°C.

At the same time, a large fraction of the maleic anhydride transfers from the gas into the circulating liquid.

This means the section has to handle two large duties simultaneously:

remove sensible heat from the process gas

and

remove the heat released as maleic anhydride is absorbed.

The liquid is withdrawn, cooled through an external heat exchanger and returned above the packing bed.

Because the recirculation flow is controlled by pumps, the liquid-to-gas ratio in this section is relatively high compared with the upper absorption zones.

That is an environment where structured packing makes good engineering sense.

The bed can provide:

• large wetted interfacial area;
• effective gas-liquid heat transfer;
• low pressure drop;
• short liquid residence time;
• open vapor passages.

The process disclosure therefore identifies structured packing or metal grids with high heat-transfer capacity as preferred internals for this cooling section.

This is not simply a distillation-style efficiency application.

Here, structured packing is partly functioning as a direct-contact heat-transfer surface.

Why a Second Packed Cooling Section Can Be Useful

After the first stage, the gas may still require additional cooling before entering the main absorption section.

The disclosed process therefore uses a second circulated cooling zone.

This second section can remove another portion of the total heat load and further reduce gas temperature toward approximately 60–75°C in the cited design. Structured packing or metal grids are again identified as preferred internals.

Why divide the cooling into two sections instead of performing one aggressive cooling step?

Because the temperature profile itself is part of maleic anhydride recovery performance.

Staged cooling gives the operator more control over:

• heat removal;
• solvent temperature;
• absorption rate;
• approach to the solids-formation limit.

That makes the packed beds part of a controlled thermal system:

packing → circulating solvent → external heat exchanger → packing

rather than isolated blocks of mass-transfer media.

For DAIER, this means an RFQ for these sections should not be reviewed only from gas and liquid flow.

The external circulation and cooling arrangement also matter.

Why “Colder Is Better” Is Wrong in This Absorber

Gas absorption usually improves as temperature decreases.

It would therefore be tempting to cool the maleic anhydride absorber as much as possible.

That can be a serious mistake.

The published recovery process identifies two lower-temperature constraints.

First, the process should avoid reaching conditions where excessive water condenses inside the absorber.

Second, decreasing temperature can reduce fumaric acid solubility and promote formation of solid deposits.

So the relationship is not:

lower temperature = always better recovery.

Instead, the tower has an operating window.

Cooling helps absorption only until chemistry and phase behavior begin creating maintenance problems.

This produces a much more realistic engineering target:

Cool enough to recover maleic anhydride efficiently, but not so far that water condensation and fumaric-acid precipitation destabilize the tower.

That temperature window is one of the most important pieces of information for selecting the packing and designing the circulation system.

Too Hot Is Also a Problem

Running the absorber hotter avoids low-temperature precipitation, but creates the opposite problem.

The disclosed industrial process notes that excessively high temperature can:

• reduce maleic anhydride absorption efficiency;
• increase conversion of maleic acid toward fumaric acid. 

This matters because water in the system can hydrolyze maleic anhydride to maleic acid.

Maleic acid can then isomerize to fumaric acid.

The resulting chemistry creates a troublesome loop:

water enters or condenses

→ maleic anhydride hydrolyzes

→ maleic acid forms

→ fumaric acid forms

→ fumaric acid can precipitate as conditions change

The packing supplier therefore cannot treat temperature as merely a material-selection number.

Temperature influences the chemistry that eventually determines fouling behavior.

Low Liquid Residence Time Has Chemical Value

Structured packing is often promoted because of low pressure drop.

In this maleic anhydride service, another property matters strongly:

low liquid residence time.

The disclosed process specifically emphasizes minimizing residence time in sections where maleic-acid-to-fumaric-acid conversion is undesirable. Compact external heat exchangers are also preferred in that process for the same reason.

This creates an important design principle.

A large liquid inventory may provide thermal buffering.

But it also keeps reactive liquid inside the system longer.

If undesired acid conversion continues during that time, excessive holdup can make the chemistry worse.

Structured packing can help because efficient contacting does not require large liquid pools on every stage.

The process value therefore becomes:

high interfacial contact with relatively low stored liquid inventory.

That is different from a normal catalogue statement about “low holdup.”

Here, liquid holdup can influence formation of a troublesome solid precursor.

So Why Not Fill the Entire Absorber With Structured Packing?

This is the most important part of the article.

Because the upper absorption section has a completely different hydraulic condition.

In the published design, the main solvent stream represents approximately 75–90% of the total solvent flow, but the upper absorption section still operates under a lower local liquid-loading regime than the heavily recirculated cooling zones.

Specially designed trays are preferred there for controlled gas-liquid contacting and liquid distribution.

The final absorption section is even more extreme.

Only a smaller clean-solvent stream—about 10–25% of total solvent flow in the disclosed process—is used to wash the remaining maleic anhydride from a large gas stream.

That creates a very high gas-to-liquid ratio.

Under such low-liquid-loading conditions, the cited process explicitly states that random or structured packing is not the preferred choice for the final section; a small number of specially designed trays, including bubble-cap designs, are favored.

This is exactly the engineering boundary generic supplier pages often miss:

Structured packing needs enough liquid irrigation to develop the wetted area assumed in its mass-transfer performance.

If liquid flow becomes extremely low, achieving reliable distribution over a full packing cross-section becomes increasingly difficult.

The packing may have enormous geometric surface area.

But unwetted surface provides little absorption benefit.

The Same Tower Can Therefore Need Packing and Trays for Opposite Reasons

A useful way to visualize the disclosed absorber is:

Cooling Zone C1High recirculation liquid load→ structured packing / grid attractive

Cooling Zone C2High recirculation + controlled final cooling→ structured packing / grid attractive

Main Absorption ZoneLower liquid loading + high separation requirement→ special trays favored in the cited process

Final Polishing ZoneVery low liquid load + very high gas/liquid ratio→ trays favored

This is not an argument that trays are better than structured packing.

It is evidence that internals should follow local hydraulic conditions.

A hybrid column can outperform a tower filled entirely with one supposedly “high-performance” internal because each technology operates where its physical strengths are useful.

Fumaric Acid Changes the Meaning of “Fouling Margin”

For ordinary clean-gas absorption, fouling margin may primarily mean allowing enough open area for small amounts of dirt.

Maleic anhydride recovery is more complicated because solids can be generated by process chemistry.

If fumaric acid precipitation begins inside a structured packing bed, deposits may initially appear in colder locations.

Those deposits can then:

• block corrugation channels;
• disturb liquid spreading;
• increase local vapor velocity;
• raise pressure drop;
• create dry and overloaded regions.

The resulting maldistribution can make temperature control worse.

So the failure mechanism can become self-reinforcing:

temperature excursion

→ solid formation

→ maldistribution

→ poorer heat/mass transfer

→ larger temperature deviation

For this reason, choosing an extremely fine packing only for maximum surface area may not be appropriate if the process operates close to a crystallization boundary.

An open geometry and clean drainage path may be more valuable.

The Rich Solvent Also Needs Water Management

The disclosed recovery configuration includes a stripping zone below the reaction-gas feed.

Its purpose is not primarily to recover the final purified maleic anhydride.

Instead, hot air or low-humidity inert gas removes part of the water carried by the rich solvent before that liquid leaves the absorber.

This matters because reducing water helps limit formation of maleic acid and, subsequently, fumaric acid.

So the full absorber is solving four coupled problems:

absorption

gas cooling

water control

solid-formation control

A packing supplier who looks only at the absorption equilibrium is missing most of the equipment problem.

A High-Efficiency Packing Can Still Be the Wrong Packing

Suppose two structured packings are being considered.

Packing A has:

• smaller corrugations;
• greater specific surface area;
• higher expected clean-system efficiency.

Packing B has:

• larger channels;
• somewhat lower surface area;
• lower pressure drop;
• easier drainage;
• greater tolerance to deposits.

Which is better?

There is no correct answer until the tower section is identified.

For a clean polishing separation elsewhere in the plant, Packing A may be excellent.

For a maleic anhydride cooling zone operating near a fumaric-acid precipitation limit, Packing B may provide a safer long-term balance.

This is why the DAIER discussion should move beyond:

“250Y or 350Y?”

and first answer:

What is this bed actually doing—cooling, bulk absorption, polishing, or water stripping?

Only after that should packing geometry be selected.

What DAIER Should Ask for a Maleic Anhydride Absorber RFQ

A customer should ideally identify the exact column section rather than requesting only “structured packing for maleic anhydride.”

Useful process information includes:

• section duty: cooling, absorption or stripping;
• reaction-gas flow;
• maleic anhydride concentration;
• inlet gas temperature;
• water content;
• organic solvent type;
• solvent circulation rate;
• local liquid-to-gas ratio;
• inlet and outlet temperature for each section;
• allowable pressure drop;
• expected fumaric-acid content;
• known crystallization or fouling history;
• tower inside diameter;
• bed height;
• liquid distributor arrangement;
• chimney tray / collector arrangement;
• external cooler duty;
• operating pressure;
• approved material specification.

For an existing tower, three records are especially valuable:

temperature profile

pressure-drop profile

location of solid deposits

Together, these reveal whether the current problem is primarily:

• inadequate heat removal;
• poor liquid distribution;
• excessive cooling;
• solids precipitation;
• hydraulic restriction.

The Important Question Is Where Structured Packing Should Stop

Most structured-packing articles try to prove how many applications can use packing.

Maleic anhydride recovery teaches a more sophisticated lesson:

good engineering also requires knowing where structured packing should not be used.

In the disclosed absorber, structured packing performs strongly in the high-circulation cooling zones because those sections need:

large wetted area + strong direct-contact cooling + low pressure drop + low liquid residence time.

Farther up the same tower, very different liquid loads favor specially designed trays.

At the same time, temperature has to remain between two undesirable limits:

too hot → weaker absorption and more acid isomerization

too cold → condensation and fumaric-acid solid deposition

The useful engineering question is therefore:

“At each elevation of the maleic anhydride absorber, what are the local liquid load, heat-removal duty and solids risk—and does structured packing still operate inside its hydraulic advantage there?”

That question can lead to a hybrid absorber that is much more reliable than filling the whole vessel with a single internal simply because it has the highest quoted efficiency.