Large-scale manufacture of C8 and C10 plasticizer alcohols for flexible PVC relies on the base-catalysed self-aldolization of n-butyraldehyde or n-valeraldehyde followed by catalytic hydrogenation of the resulting α,β-unsaturated aldehyde. The reaction sequence converts two molecules of C4 aldehyde into one molecule of 2-ethyl-2-hexenal and water, then hydrogenates the unsaturated aldehyde with two moles of hydrogen to 2-ethylhexanol; the analogous C5 sequence produces 2-propylheptanol from n-valeraldehyde via 2-propyl-2-heptenal. The route is integrated with oxo aldehyde production and consumes hydrogen from the same reforming or steam-cracker off-gas system. In the aldol stage, aqueous sodium hydroxide at 2–5 wt% NaOH is dispersed in the aldehyde phase at 90–130°C and 0.3–1.0 MPa; the organic condensate is decanted, washed, and distilled to recover unreacted aldehyde. In the hydrogenation stage, the unsaturated aldehyde is contacted with hydrogen over a copper-based fixed-bed catalyst in a multi-tubular reactor at 130–180°C and 2.5–4.0 MPa. The product alcohol is then purified by vacuum distillation to a purity of at least 99.0 wt%, because downstream esterification with phthalic anhydride or terephthalic acid is sensitive to carbonyl impurities, water, and acid number. 2-Ethylhexanol has a molecular weight of 130.23 and a theoretical hydroxyl number of 430.8 mg KOH/g; 2-propylheptanol has a molecular weight of 158.28 and a theoretical hydroxyl number of 354.5 mg KOH/g.

What Limits Liquid Hourly Space Velocity in Trickle-Bed Hydrogenation of 2-Ethyl-2-Hexenal?

In trickle-bed hydrogenation of the aldol intermediate, liquid hourly space velocity, defined as volumetric liquid feed per hour divided by catalyst bed volume, is controlled primarily by hydrogen mass transfer and heat removal rather than intrinsic catalyst activity. For a commercial copper-zinc oxide-alumina catalyst with extrudate diameter 3.0 mm and length 5.0 mm, a typical LHSV is 0.2–0.8 h⁻¹ when the reactor is operated at 2.5–4.0 MPa hydrogen partial pressure and 130–180°C. At LHSV values above 0.8 h⁻¹, the concentration of saturated intermediate 2-ethylhexanal in the reactor effluent rises from below 0.1 wt% to above 0.8 wt%, and the carbonyl content measured by gas chromatographic internal normalization increases proportionally. The hydrogen-to-ene molar ratio is maintained at 2.1:1 to 2.5:1, with a high-pressure recycle compressor circulating 10–20% excess hydrogen. The multi-tubular reactor contains 2,000–8,000 tubes with inside diameter 25–40 mm and length 6–9 m; heat is removed by circulating oil at 150–200°C and the maximum tube-wall heat flux is limited to 20–35 kW m⁻². If wetting efficiency falls below 70%, local hot spots above 210°C cause sintering of copper crystallites and an irreversible increase in pressure drop. Published data for the exact wetting efficiency in full-scale commercial reactors is limited; the above boundary values are extracted from typical licensor operating envelopes for intermediate aldehyde hydrogenation.

In the aldol condensation reactor, the primary process conflict is the narrow temperature window between fast phase separation and excessive high-boiler formation. n-Butyraldehyde and 2.5 wt% aqueous NaOH are contacted at 105–115°C and 0.5–0.8 MPa with residence time 20–45 min. Below 95°C, the organic droplets coalesce slowly and the decanter interface becomes difficult to control; above 120°C, aldol trimer and tetramer formation increases markedly, raising high-boiler content in the organic phase from 0.5–2.0 wt% to more than 4.0 wt%. The exothermic reaction is managed by an external recirculation loop through a shell-and-tube exchanger with recirculation ratio 20:1 to 30:1, and the exchanger outlet temperature is controlled within ±3°C of setpoint. The reactor effluent is separated in a horizontal decanter sized for a linear settling velocity of 0.2–0.5 m/h; the spent caustic is purged continuously to remove sodium carboxylate byproducts. Production-scale data show that pump impeller tip speeds above 12 m/s in the recirculation loop increase emulsification and carry-over of aqueous sodium hydroxide into the organic phase, which can elevate sodium content in the hydrogenation feed and accelerate fixed-bed catalyst deactivation. The organic condensate is water washed to pH 6.5–8.0 and dried before entering the hydrogenation section.

When Feedstock n-Butyraldehyde Purity Falls Below 99.0 wt%

Feedstock quality changes the aldol product isomer distribution and the distillation sharpness of the final plasticizer alcohol. n-Butyraldehyde entering the condensation reactor should contain at least 99.0 wt% n-butanal, with isobutyraldehyde below 0.5 wt%, water below 0.2 wt%, and C4 butanols below 0.2 wt%. When isobutyraldehyde rises to 0.5–1.0 wt%, cross-aldol condensates introduce branched C8 isomers that hydrogenate to alcohol isomers with boiling points close to 2-ethylhexanol. The distillation range broadens from 183.5–186.0°C to as much as 183.0–189.0°C at 101.3 kPa when tested according to ASTM D1078-05. In addition, residual butanols and water in the feedstock influence the hydrogenation catalyst: water above 0.2 wt% in the organic feed reduces copper catalyst activity by competitive adsorption and can promote hydrothermal loss of zinc oxide support surface area. Organic acids, if present above 0.1 wt% as butyric acid, consume sodium hydroxide and form sodium butyrate, which stabilizes emulsions in the aldol decanter and increases downstream sodium carry-over. Therefore, storage tanks for n-butyraldehyde are blanketed with dry nitrogen, and the feed is dried over molecular sieve 3A to water below 0.05 wt% before hydrogenation. Published data for the effect of specific cross-aldol isomers on PVC low-temperature flex is limited; however, the observed broadening of the alcohol distillation curve is sufficient to reduce esterification batch-to-batch consistency.

After hydrogenation, the reactor effluent contains 2-ethylhexanol, light ends, water, dissolved hydrogen, and C12 or C16 high-boiling condensation products. The effluent is cooled to 40–60°C in a high-pressure separator at 2.5–4.0 MPa; the gas phase is scrubbed with recycled liquid alcohol to remove olefinic intermediates and compressed for reuse. The liquid is depressurised to 0.05–0.10 MPa and fed to a light-ends column with 20–30 theoretical stages, operated at a reflux ratio of 0.5–1.0. Overhead vapor contains water, unreacted aldehyde, and low-boiling hydrocarbons; the bottoms are transferred to a vacuum product column operated at 10–30 kPa absolute with 40–60 theoretical stages. The product 2-ethylhexanol is withdrawn as a side stream with purity 99.0–99.8 wt%, color ≤ 10 Pt-Co, water ≤ 0.05 wt%, and acid number ≤ 0.01 mg KOH/g. High-boiling compounds are rejected in a wiped-film evaporator at 130–150°C and 1–5 kPa absolute; their viscosity may exceed 200 mPa·s at 50°C, requiring heated gear pumps and traced transfer lines. The heavy stream can be sent to fuel blending or thermal oxidation; recycle back to the aldol reactor is limited to 5 wt% of total feed because aldol trimer accumulation increases the organic phase viscosity and reduces decanter separation efficiency.

Vacuum Distillation Column Hydraulics and High-Boiler Recycle

Product distillation columns in plasticizer alcohol service are subject to fouling from thermally labile high-boiling aldol condensates. Structured packing with a specific surface area of 250–500 m²/m³ is specified for the rectifying section, while the stripping section uses fouling-resistant internals such as sieve trays or dual-flow trays. The column overhead pressure is maintained at 10–20 kPa absolute with steam ejectors and liquid-ring vacuum pumps; the bottom temperature is held at 150–165°C to limit dehydration of the alcohol to ethers and olefins. Falling-film reboilers operate with a recirculation ratio of 3:1 to 5:1 and a maximum tube skin temperature of 170°C; tube skin temperatures above this threshold increase polymer formation and raise reboiler pressure drop by 0.5–1.5 kPa per month. If the high-boiler content in crude alcohol rises above 5 wt%, the product column requires a reflux ratio increase from 1.0 to 1.8 or more, and the reboiler steam consumption per tonne of product increases by approximately 15–20%. Batch-to-batch changes in high-boiler load caused by aldol temperature excursions of +3°C are measurable as a shift in the lower product tray temperature by 2–5°C and as a slight increase in product color from ≤ 5 Pt-Co to ≤ 15 Pt-Co. The high-boiler purge flow is therefore controlled by mass balance every 4 h, not by fixed volumetric setpoint, to maintain stable column hydraulics.

Plasticizer alcohol quality is monitored with a combination of chromatographic, titrimetric, and spectrophotometric methods because downstream esterification and PVC compounding are sensitive to trace carbonyl, water, and acid impurities. Purity is determined by capillary gas chromatography with FID using a 30 m × 0.25 mm low-polarity column and internal normalization; the method separates unreacted aldehyde, unsaturated aldehyde intermediate, saturated aldehyde intermediate, product alcohol, and C12 high-boiling species. Hydroxyl number is determined by phthalation according to DIN 53240-2; the theoretical value for pure 2-ethylhexanol is 430.8 mg KOH/g. Water is measured by Karl Fischer coulometric titration according to ASTM E203-16. Color is measured by platinum-cobalt scale according to ASTM D1209-05. Acid number is measured by titration with methanolic potassium hydroxide according to ASTM D1613-17. Distillation range is measured according to ASTM D1078-05. Table 1 lists the typical specification limits for 2-ethylhexanol used in phthalate ester production.

Thermal Degradation Pathways in Copper-Based Hydrogenation Catalysts

Copper-based hydrogenation catalysts are activated from the oxide form by controlled reduction with dilute hydrogen at 180–220°C; the reduction exotherm must be limited to 10–15 K h⁻¹ until the reduction front passes through the bed, because local temperatures above 230°C cause premature copper sintering. Fresh copper-zinc oxide-alumina catalysts typically contain 30–45 wt% CuO and have a BET surface area of 60–120 m²/g; after activation, the active copper surface area is 20–40 m²/g. During normal operation at 130–180°C, the desired hydrogenation of the C=C bond and the carbonyl group proceeds with an overall reaction enthalpy of approximately -190 to -220 kJ mol⁻¹ for the two-step conversion of 2-ethyl-2-hexenal to 2-ethylhexanol. Hot spots above 220°C shift selectivity toward decarbonylation and aldol-related side products, and the resulting carbon deposits bridge catalyst pellet interstices, increasing reactor pressure drop by 0.5–1.0 bar over several months. Sulfur compounds in the aldehyde feed must be kept below 1 ppm because copper-based catalysts are irreversibly poisoned; the hydrogenation catalyst is therefore incompatible with feedstocks originating from mercaptan-contaminated streams. Once the exit carbonyl content exceeds 0.10 wt% at the design LHSV, the catalyst bed is regenerated by controlled oxidation at 350–400°C or replaced. Published data for the exact crystallite size distribution in used commercial fixed-bed catalysts is limited; the above ranges are consistent with vendor technical bulletins and industrial operating manuals for aldehyde hydrogenation.

The distilled alcohol is stored in 304L stainless steel or internal-coated carbon steel tanks with dry nitrogen blanketing at 0.5–2.0 kPa gauge; moisture ingress is maintained below 0.05 wt% because water hydrolyzes titanium- or tin-based esterification catalysts and increases final plasticizer acidity. When ambient relative humidity exceeds 60%, the alcohol is pre-dried through molecular sieve 3A before esterification. Esterification with phthalic anhydride is carried out in a glass-lined or 316L stainless steel reactor at 185–210°C and reduced pressure of 30–80 kPa absolute with a molar ratio of alcohol to anhydride of 2.2:1 to 2.5:1. The reactor overhead system removes water through a packed column and condenser; excess alcohol is stripped at 150–170°C and 5–10 kPa absolute and recycled. The resulting diester is dried to water ≤ 0.05 wt% and filtered through a 5 μm bag filter. In flexible PVC processing, the phthalate or terephthalate ester is mixed with suspension-grade PVC resin, calcium-zinc or barium-zinc stabilizers, and lubricants in a hot mixer at 800–1,200 rpm, then processed in a counter-rotating twin-screw extruder with L/D 40:1–44:1 and melt temperature 160–195°C. Plasticizer compatibility under compression is evaluated according to ASTM D3291-11, and plasticizer sampling and testing are performed according to ASTM D1045-19. Table 2 compares the two principal plasticizer alcohol types produced by the two-stage aldol hydrogenation sequence.

Safe Operating Boundaries for Aqueous Caustic Neutralisation and Waste Handling

Spent caustic discharged from the aldol decanter contains residual sodium hydroxide, sodium carboxylates, emulsified organic aldehyde, and colored high-boiling condensation products. It is neutralized with diluted sulfuric acid at 10 wt% in a recirculating neutralization loop; pH is maintained at 6.5–8.5 with inline glass electrodes and automatic temperature compensation. The neutralization is exothermic and the loop temperature is held at 40–60°C by a water-cooled shell-and-tube exchanger. The treated aqueous stream is then routed to biological wastewater treatment; feed to the biotreater must contain less than 500 mg/L chemical oxygen demand and less than 10 mg/L free organics to avoid sludge bulking. Hydrogen vent streams from the high-pressure separator and hydrogen compressor seals are collected in a closed flare header; the lower flammable limit of hydrogen in air is 4.0 vol%, and area classification follows IEC 60079-10-1. Oxygen concentration in the hydrogen feed is continuously monitored and must remain below 0.5 vol% to avoid flammable mixtures in the high-pressure loop. The aqueous caustic neutralization system is interlocked to stop the aldol reactor feed pump if pH exceeds 9.0 or drops below 5.0, preventing discharge of unneutralized waste.