In solvent-free continuous oxidation of n-butyraldehyde to n-butyric acid, the process design is built around the competing kinetics of acyl radical initiation, oxygen insertion, and peroxybutyric acid decay. The reaction proceeds through a radical-chain sequence in which the aldehyde is oxidized by molecular oxygen to the carboxylic acid, and the intermediate peroxy acid is both the chain carrier and the principal thermal hazard. Industrial trains retain a butyric acid heel from the previous production campaign because the acid product is a better polar protic solvent for the radical intermediates than the aldehyde itself, and this internal solvent use eliminates the need to recover an external solvent such as acetic acid or propionic acid from the oxidation mixture. The liquid-phase density of n-butyraldehyde is approximately 0.80 g·cm⁻³ at 20 °C, while butyric acid has a density of 0.9528 g·cm⁻³ at the same temperature, and the difference alters hydrostatic pressure at the bottom of a 10 m liquid-full reactor by more than 0.015 MPa when the acid fraction rises from 10 wt% to 90 wt%. Because the oxidation is exothermic, the removal of external solvent reduces the thermal mass of the reactor contents and increases the cooling load per unit reactor volume; therefore heat-transfer surfaces are sized for a specific heat release of 290–350 kJ·mol⁻¹ of aldehyde converted, with circulated cooling water or an evaporative condenser on the reactor vent. The oxidant is compressed air or oxygen-enriched air, and the partial pressure of oxygen is controlled so that the dissolved oxygen concentration at the liquid outlet remains below the threshold that permits peroxybutyric acid accumulation above 0.5 wt%. In air-fed systems, the oxygen content in the headspace is kept below 10 vol% on a wet basis, and the limiting oxygen concentration for n-butyraldehyde-air-nitrogen mixtures is measured by ASTM E681-04 or EN 1839:2017. The shutdown logic that closes the air feed and opens nitrogen inerting is configured as a safety instrumented function under IEC 61511, with independent oxygen analyzers and a fail-closed oxidant control valve.

The substitution of added solvent by internally recycled butyric acid changes the concentration profile across a two- or three-stage continuous stirred tank cascade. In the first stage, the aldehyde feed is diluted with returned acid to keep the unreacted aldehyde concentration below 50 wt%, which reduces the severity of a runaway oxidation because the aldehyde is the main fuel. In the final stage, the aldehyde concentration is driven below 0.5 wt% by using higher oxygen partial pressure and longer residence time, and the crude product is sent to a stripping column where residual aldehyde and water are removed overhead. Published data for this specific solvent-free configuration is limited, but the operating window is constrained by the flash point of the aldehyde-rich feed, which is below −7 °C, and by the flash point of the acid-rich recycle, which is approximately 72 °C. The lower explosion limit of n-butyraldehyde in air is below 3 vol%, and the upper explosion limit is approximately 12.5 vol%; therefore all storage and feed tanks require nitrogen blanketing and explosion-proof electrical classification in accordance with local fire codes and NFPA 69. The reactor agitator in the first CSTR is specified for gas dispersion rather than simple blending because the oxygen mass transfer coefficient must be maintained above 0.08 s⁻¹ when the liquid-phase viscosity increases with acid concentration. A gas-inducing impeller or a dual-impeller configuration with a Rushton turbine lower stage and a pitched-blade turbine upper stage provides the necessary gas hold-up, and the agitator power per unit volume is typically in the range 1.0–2.5 kW·m⁻³ depending on the gas flow rate and liquid height-to-diameter ratio.

How Does Peroxybutyric Acid Accumulation Change Oxygen Mass Transfer Requirements in the Final Oxidation Stage?

The peroxybutyric acid concentration in the final oxidation stage is not simply a function of temperature; it depends on the balance between oxygen mass transfer, radical initiation, and the aldehyde concentration available for peracid reduction. When the aldehyde concentration falls below 2 wt%, the rate of peracid reaction with aldehyde decreases, and the peracid concentration can rise if the oxygen supply is not reduced in parallel. This creates a narrow processing window: the dissolved oxygen must remain high enough to complete aldehyde conversion above 90%, but low enough to prevent net peracid accumulation. In a bubble column or stirred reactor operating at 55 °C, the dissolved oxygen concentration is usually maintained below 1.0 mg·L⁻¹ at the liquid outlet, and the peroxybutyric acid concentration is kept below 0.5 wt% by manipulating the oxygen partial pressure in the feed gas. The mass transfer coefficient k_La is influenced by liquid viscosity, ionic strength, and the presence of surface-active acid products; as the acid concentration increases from 20 wt% to 80 wt%, the liquid-phase dynamic viscosity can rise from about 0.5 mPa·s to about 1.0 mPa·s at 55 °C, which decreases the liquid film diffusion coefficient and reduces k_La by 10–20% unless agitator speed or gas velocity is increased. The final oxidation stage therefore requires independent oxygen trim control on the spent oxidant and a liquid-phase peroxide analyzer rather than a fixed air-to-aldehyde ratio.

The mechanical design of the final reactor must accommodate a gas-liquid dispersion that shifts from bubbly flow to coalesced flow as the aldehyde concentration drops and the acid concentration rises. In coalesced flow, large bubbles rise through the liquid with reduced interfacial area, and the oxygen transfer rate can become the process bottleneck. To maintain the required dissolved oxygen profile, the final reactor is equipped with a top-entering agitator with a tip speed below 4.5 m·s⁻¹ to avoid excessive shear degradation of the peracid, while the gassed-to-ungassed power ratio is kept above 0.6 by using a disk-style impeller below the liquid surface. The reactor is usually baffled, but the clearance between baffles and vessel wall is increased to 15–25 mm to minimize dead zones where acid can stratify. The oxygen partial pressure in the final stage is limited by the flammability envelope of the aldehyde-rich vapor; with air as the oxidant, the oxygen partial pressure is generally below 0.12 MPa, and the total pressure is selected so that the oxygen concentration in the vent remains below 10 vol% after accounting for aldehyde and water vapor dilution.

At production rates above 20,000 t·a⁻¹ of finished butyric acid, the air-fed oxidation train is often integrated with a thermal oxidizer rather than a conventional carbon adsorption vent-recovery system. The off-gas from the reactor contains unreacted n-butyraldehyde, butyric acid mist, carbon dioxide, nitrogen, and water vapor, and its condensation temperature is set to recover the bulk of the aldehyde and acid while leaving a combustible gas stream that is destroyed at 850 °C with a residence time above 0.5 s in accordance with local waste-gas incineration permits. The liquid recovered from the vent condenser is returned to the first oxidation stage, but the water content of the recycle must be controlled because water partitions into the acid phase and can suppress the aldehyde oxidation rate by diluting the organic substrate and by hydrogen bonding with the peracid intermediate. The water content of the recycled acid is monitored by ISO 760-1978 Karl Fischer titration and is maintained below 0.5 wt% by a side-stream distillation. The recycled acid also contains dissolved sodium or manganese salts from the catalyst system, and these salts can precipitate in the heat exchanger if the acid concentration is allowed to exceed 95 wt% at low temperature. The catalyst is removed from the crude acid by water washing or by distillation, but the exact removal step depends on whether the downstream application requires food-grade butyric acid or technical-grade acid for ester synthesis.

Vacuum stripping is used after the oxidation cascade to remove residual aldehyde and water before the acid enters the product column. The stripping column operates at 10–20 kPa absolute and a bottom temperature below 120 °C to avoid thermal decomposition of peroxybutyric acid, which can occur rapidly above 70 °C if the peroxide is concentrated. The reboiler is typically a falling-film or forced-circulation type because a thermosyphon reboiler can allow wall temperatures to exceed the decomposition threshold when the tubes are partially dry. The column overhead is condensed and phase-separated, and the organic phase is returned to the oxidation reactor while the aqueous phase is sent to wastewater treatment after neutralization. The crude butyric acid leaving the stripping column bottom has an acid number near 630 mg KOH·g⁻¹ when the product is dry, and this value is confirmed by ASTM D664-18. The residual aldehyde concentration is checked by gas chromatography with a flame ionization detector, and the target is below 0.1 wt% for most esterification applications.

If Butyric Acid Is Recycled as an Internal Solvent in a Thermal Oxidizer-Integrated Loop

When internal butyric acid recycle is selected as the solvent mode, the entire liquid-handling system must be designed for a fluid whose viscosity and density change continuously with temperature and acid concentration. The recycle pump is a magnetic-drive centrifugal unit with a casing constructed from 316L stainless steel or high-alloy austenitic stainless steel, and the impeller clearance is set to minimize heat input that could lead to localized peracid decomposition. The pump is sized to deliver a recycle flow rate equal to 30–60% of the fresh aldehyde feed mass flow, and the discharge pressure is typically 0.4–0.8 MPa above the reactor operating pressure to provide positive injection into the first oxidation stage. Because the recycled acid contains residual dissolved oxygen and traces of peroxybutyric acid, the pump casing is vented to the flare or thermal oxidizer and the mechanical seal is a double cartridge type with a barrier fluid maintained at a pressure 0.15–0.25 MPa above the pump discharge pressure. The barrier fluid is selected from a perfluoropolyether or a food-compatible polyalkylene glycol, and its pressure is monitored by a differential pressure transmitter tied to the pump interlock.

Thermal integration between the oxidation reactors and the downstream distillation columns is arranged so that the heat released by the oxidation can preheat the recycle acid before it enters the first reactor. This preheat exchanger is a plate-and-frame unit with thin titanium or Alloy C-276 plates, and the hot-side approach temperature is kept below 15 °C to prevent film boiling on the acid side. The oxidation reactor itself is cooled by an external shell-and-tube exchanger with the process fluid on the tube side and cooling water on the shell side, and the liquid circulation rate is set to maintain a tube-side velocity above 1.5 m·s⁻¹ to reduce fouling by catalyst residues and acid-degradation polymers. The heat exchanger is protected from over-temperature by a high-priority interlock that closes the air feed and ramps the cooling water flow to maximum when the reactor temperature exceeds the setpoint by more than 5 °C. This narrow margin reflects the process safety consequence of a runaway peracid decomposition, which can raise the pressure rise rate above the relief valve capacity if the thermal excursion is not interrupted.

Because n-butyric acid at 95 wt% purity is corrosive to carbon steel, the entire hot acid circuit downstream of the reactor is specified in 316L stainless steel with a minimum molybdenum content of 2.0 wt%, and the material is tested for intermetallic precipitation by ASTM A923-14 when welding is performed. The weld procedures are qualified to ASME BPVC Section IX, and post-weld pickling is specified to remove heat tint that would otherwise reduce the passive chromium oxide layer. The corrosion allowance on vessels and piping is set at 1.5–3.0 mm depending on the acid concentration and temperature, but the allowance is not a substitute for proper material selection in high-turbulence zones such as pump casings, orifice plates, and thermowell tips. In these zones, Alloy C-276 or titanium is used because the pitting resistance equivalent number of 316L is insufficient when chlorides are present above 100 ppm in the process water. The pitting resistance of the selected alloy is qualified by ASTM G48-11 in the as-welded condition, and the test is conducted at the maximum process temperature plus a 5 °C margin. The acid storage tanks are blanketed with nitrogen, and the vent lines are heat traced above 10 °C to prevent butyric acid solidification, because butyric acid freezes at approximately −5 °C.

Chloride contamination is a critical variable because butyric acid produced from municipal water or from a catalyst containing chloride can promote stress corrosion cracking in hot 316L equipment. The chloride content of the process water is monitored by ion chromatography and is maintained below 50 ppm in the final product column feed. Where chloride cannot be reduced, the column internals and reboiler tubes are upgraded to Alloy C-276 or Grade 2 titanium, and the use of austenitic stainless steel fasteners is prohibited. The acid transfer lines are welded rather than flanged in high-risk sections, and the flanged connections that remain are specified with spiral-wound gaskets containing a flexible graphite filler and an inner ring of 316L. This reduces the leak potential from thermal cycling, because the differential thermal expansion between the pipe and the support structure can exceed 1.5 mm·m⁻¹ when the line heats from 20 °C to 120 °C.

Acid Number, Peroxide Value, and Aldehyde Carbonyl Titration Disclose Incomplete Oxidation

The quality of the crude butyric acid is not judged solely by gas chromatography; the acid number, peroxide value, water content, and residual aldehyde concentration together define whether the oxidation step has achieved the required conversion and whether the downstream refining train can meet a commercial specification. The acid number is measured by ASTM D664-18, and for dry butyric acid the value is approximately 638 mg KOH·g⁻¹. A lower acid number indicates the presence of water or non-acid organic impurities, while a higher value indicates the presence of a lower-molecular-weight carboxylic acid such as propionic acid. The water content is measured by ISO 760-1978, and the target after distillation is below 0.2 wt% for storage stability. The peroxide value is determined by iodometric titration, and the result is expressed as peroxybutyric acid concentration; the target after the stripping column is below 0.05 wt% because higher values can cause downstream discoloration and can react with unsaturated impurities in the esterification feed. The residual aldehyde concentration is measured by gas chromatography with a flame ionization detector or by derivatization with 2,4-dinitrophenylhydrazine, and the target is below 0.1 wt% as n-butyraldehyde. The color of the product is measured by ASTM D1209-05 and is maintained below 15 APHA units for technical-grade acid, while food-grade material may require a carbon bed or a finishing distillation to meet a lower color target.

These measurements are not independent; a high acid number combined with a high aldehyde concentration indicates that the oxidation train is either under-sparging oxygen or operating at too low a temperature, while a high peroxide value with a low aldehyde concentration indicates that the final reactor is being operated with excessive oxygen partial pressure or insufficient residence time for peracid decomposition. The control strategy therefore uses a model-based cascade that adjusts the air flow to the final reactor based on the measured aldehyde concentration in the crude acid, while the peroxybutyric acid concentration is controlled by a separate trim loop that adjusts the final reactor temperature within a narrow band of 3–5 °C. This dual-loop strategy is implemented in a distributed control system with online analyzers for oxygen, water, and acid number, but the final product release is always based on laboratory measurements because the online acid number analyzer has a measurement lag of 15–30 min and can drift when the acid contains dissolved carbon dioxide.

In the product distillation column, the separation between butyric acid and heavy byproducts such as butyric anhydride and aldol condensation products is achieved at a reflux ratio between 1.0 and 2.0, with the column operated at 10–50 kPa absolute to keep the reboiler temperature below 160 °C. The column is packed with structured packing instead of trays because butyric acid has a relatively low surface tension and the packing provides a higher number of theoretical stages per meter without excessive pressure drop. The overhead of the product column is a mixture of water, residual aldehyde, and low-boiling esters, and it is condensed in a shell-and-tube condenser with the process stream on the tube side and chilled water on the shell side. The aqueous phase is separated from the organic phase in a horizontal decanter sized for a residence time of 30–45 min, and the organic phase is returned to the oxidation reactors as additional internal solvent. The bottom product is cooled through a plate-and-frame heat exchanger before entering the acid storage tank, and the cooling water temperature is controlled to keep the acid outlet above 5 °C to avoid freezing in the downstream piping and maintain pumpability.

The reboiler of the product column is a forced-circulation unit with a circulation pump that maintains a tube-side velocity above 2.0 m·s⁻¹, and the tube wall temperature is kept below 170 °C to minimize the formation of acid anhydride and color bodies. The reboiler tubes are made of Alloy C-276 or titanium, and the heating medium is high-pressure steam at 0.6–1.2 MPa with a desuperheater to prevent dry-wall conditions. Fouling in the reboiler is monitored by the pressure drop across the tube side, and a 10% increase in pressure drop triggers a cleaning cycle. Published data for the fouling rate of this specific reboiler configuration is limited, but operating records from ester-grade butyric acid plants indicate that the reboiler can be operated for 6–12 months between chemical cleanings if the crude acid is water-washed to reduce catalyst residues before distillation.

Butyric Acid Refining Column Material Selection and Reboiler Fouling Chemistry

The product column and its associated reboiler are subjected to hot organic acid corrosion, trace chloride attack, and fouling from metal-organic residues that survive the upstream water wash. The material selection for the column shell is typically 316L stainless steel with a minimum molybdenum content of 2.0 wt%, but the trays or packing supports in the sections where the acid concentration exceeds 80 wt% are upgraded to Alloy C-276 because the passive film on 316L is destabilized by the combination of organic acid and water at temperatures above 100 °C. The column is designed for full vacuum operation, and the relief valves are sized for loss of cooling water and external fire exposure according to API 521 and ISO 4126-1. The reboiler tubes are specified as seamless cold-worked Alloy C-276 with a wall thickness of 1.65 mm, and the tubesheet is clad with Alloy C-276 on the process side. The reboiler inlet and outlet nozzles are lined with titanium to resist erosion-corrosion, because the circulation pump can generate localized velocities above 3.0 m·s⁻¹ at the nozzle entrances.

Fouling chemistry in the hot acid circuit is dominated by the precipitation of manganese and cobalt soaps, which form when the carboxylic acid reacts with the metal salts of the oxidation catalyst. These soaps are soluble in hot dilute acid but become insoluble as the acid concentration rises above 90 wt% and the temperature falls below 60 °C. The water wash step is designed to remove the water-soluble metal ions before the acid enters the product column, and the wash water pH is adjusted to 3.0–4.5 using a small amount of butyric acid to avoid emulsion formation. The washed acid is then dried by azeotropic distillation, and the resulting water content is below 0.1 wt% before the acid enters the product column. This water content is critical because water in the column feed increases the reboiler temperature at a given pressure and can hydrolyze ester impurities to form additional organic acids that contribute to color and odor.

For a solvent-free butyric acid train that serves both technical-grade and food-grade markets, environmental and product-compliance obligations are defined by the intended use of the acid. Technical-grade material sold for chemical synthesis is typically controlled by a technical data sheet and a REACH registration dossier, while food-grade material used as a flavoring substance is controlled under FDA 21 CFR 172.515 and the relevant food-additive purity criteria. The oxidation plant is a covered process under the local implementation of the Seveso III directive in the European Union because n-butyraldehyde is a flammable liquid, and the butyric acid inventory may exceed the threshold for toxic or environmentally hazardous substances depending on the classification. The release of butyric acid to air is governed by local odor regulations because the compound has a low odor threshold, and the thermal oxidizer must achieve a destruction and removal efficiency above 99% for organic carbon. The wastewater stream from the water wash and overhead decanter is neutralized with sodium hydroxide to a pH between 6.5 and 8.5 before discharge, and the chemical oxygen demand is reduced by biological treatment if the total organic carbon exceeds 200 mg·L⁻¹.

The compliance matrix below summarizes the critical measurement points and the standard methods that are applied to the finished acid and the process safety system.