Tert Butyl Alcohol

Catalytic Vapour-Phase Dehydration of TBA to Polymer-Grade Isobutylene: Azeotrope Recovery, Space Velocity, and Coking Thresholds

In fixed-bed tubular reactors with a single-pass pressure drop of 0.3–0.8 bar, TBA feed is dried to 0.1 wt% water before entering gamma-alumina or silica-modified alumina beds. The feed is preheated to 300–320 °C in a shell-and-tube exchanger; the catalyst bed is maintained at 340–420 °C with liquid hourly space velocity of 0.5–2.0 h⁻¹. TBA-water forms a minimum-boiling azeotrope at 79.9 °C and 11.8 wt% water, so atmospheric distillation alone cannot produce low-water feed from wet recycle streams; a molecular sieve dryer or extractive distillation step is required before catalyst contact. On production-scale units, the overhead TBA-water condensate from the azeotrope column is phase-split and the organic layer returned to a dehydration unit; failure to maintain the reflux ratio in the range 0.7–1.2 raises the water content of the recycle TBA above 0.5 wt%, and the resulting competitive adsorption on Lewis acid sites suppresses conversion. Isobutylene product for butyl rubber and polyisobutylene must be handled in a closed system with low oxygen and diene content; gas-phase oxygenates are monitored by GC-FID with methanizer according to ASTM D7423, and total dienes are controlled below 5 ppmv. The dominant side reactions are dimerization and oligomerization to C₈–C₁₂ hydrocarbons above 450 °C; these compounds deposit on the outlet ferrules and downstream partial condensers. De-coking is performed with steam/air at 480 °C, not exceeding 550 °C, because higher temperatures promote gamma-to-alpha alumina phase inversion and irreversible loss of surface area. Regeneration air supply is maintained to ISO 8573-1 class 1.2.1 for water and particulate removal. Terminal products from the isobutylene include butyl rubber produced by cationic copolymerization with isoprene at −95 °C, polyisobutylene with viscosity-average molecular weight from 40,000 to 5,000,000 g/mol, and tert-butyl phenolic antioxidants after subsequent alkylation.

The alkylation of p-cresol to 2,6-di-tert-butyl-4-methylphenol (BHT, E321) in a continuous stirred-tank reactor train is controlled not by TBA conversion but by the residence time distribution of recycled mono-tert-butyl intermediates. Feed p-cresol and dried TBA are introduced at a molar ratio of 2.1:1 to 2.5:1 TBA:p-cresol; sulfuric acid at 0.5–1.5 wt% relative to p-cresol is metered into the first reactor. The jacket temperature is held at 70–85 °C, and the TBA-water azeotrope generated during alkylation is withdrawn from the overhead condenser to prevent water accumulation above 0.2 wt% in the liquid phase. When water exceeds this threshold, the acid activity is diluted and the yield of mono-alkylated 2-tert-butyl-4-methylphenol rises; this intermediate is recycled to the second alkylation reactor after phase separation. The crude organic phase is neutralized with 10 wt% sodium carbonate, washed with deionized water until sulfate ash is below 0.05 wt%, and distilled under vacuum. Recrystallization from hexane gives BHT with a melting point of 69–70 °C and purity above 99.0 wt%. For food-contact polymer stabilization, the finished BHT is controlled under 21 CFR 178.2010 and 21 CFR 177.1520 for polyolefin applications; the specific migration limit in the European framework is 3.0 mg/kg under EU Regulation (EU) 10/2011. Terminal products include BHT-stabilized low-density polyethylene films, mineral-oil transformer fluids, and styrene-butadiene rubber. The production-scale bottleneck is the vacuum distillation of the crude alkylate; if the reboiler temperature exceeds 150 °C during stripping, sulfonate ester residues decompose and raise sulfur content in the finished antioxidant.

What Limits Esterification Yield in Tert-Butyl Acetate Production When Acetic Acid Is Recovered by Liquid-Liquid Extraction?

In a reactive distillation column with 20–35 theoretical stages, TBA and glacial acetic acid are fed in a bottom-section configuration to suppress the competing dehydration of TBA to isobutylene. The acetic acid:TBA molar ratio is kept between 1.2:1 and 1.8:1, and the sulfonated polystyrene resin catalyst bed is located in the reflux zone to maintain the catalytic temperature window of 60–100 °C. Water is removed overhead as a tert-butyl acetate/water heteroazeotrope; the organic phase is returned to the column while the aqueous phase is sent to extraction for acetic acid recovery. In production practice, the reflux ratio is held between 2.0 and 3.0 to achieve tert-butyl acetate purity above 99.0 wt% with residual acetic acid below 0.05 wt%. If the overhead pressure is not regulated within 90–110 kPa, the TBA-water azeotrope overlaps with the ester-water azeotrope and the organic-phase recovery becomes unstable; this is the most common cause of acid breakthrough into the distillate. The finished solvent is tested for distillation range under ASTM D1078, flash point under ASTM D56, and VOC content under ASTM D2369. Water content is controlled below 0.05 wt% by Karl Fischer titration ASTM E203. Tert-butyl acetate is subsequently formulated into high-solids alkyd primers, nitrocellulose lacquers, and flexographic inks; the ester-to-alcohol ratio in the final blend is adjusted to maintain the Hansen solubility boundary of the resin. The terminal products are used where medium evaporation rate and resin compatibility are required; the boiling point of tert-butyl acetate is 97.9 °C, which places it between ethyl acetate and butyl acetate in coalescing performance. Acid-resistant construction is necessary because acetic acid carryover into the top section accelerates corrosion of copper condenser tubes; therefore, condensers are fabricated from 316L stainless steel or graphite block.

When TBA is oxidized with hydrogen peroxide in a three-phase stirred reactor, the rate-limiting step shifts from peroxide conversion to the removal of heat generated by the acid-catalyzed formation of tert-butyl hydroperoxide. The feed molar ratio is maintained at 1.0:1 to 1.2:1 hydrogen peroxide:TBA, and sulfuric acid is fed at 0.5–2.0 wt% of the total liquid charge to hold the aqueous phase pH between 3.5 and 5.0. The reactor jacket is controlled at 40–60 °C; above 65 °C, the rate of TBHP self-decomposition rises rapidly, and the pressure-relief system must be sized for the onset temperature of 70 °C. Production units use 316L stainless steel or glass-lined vessels that have been passivated with 10 wt% citric acid and rinsed to remove trace iron oxides. The organic phase is separated and the aqueous layer is returned to the reactor after extraction of unreacted TBA with methyl tert-butyl ether. The crude TBHP is stabilized with sodium carbonate to a pH of 4.0–5.0 and stored below 40 °C in vented containers; organic peroxide storage is governed by NFPA 432, and transport is classified under Division 5.2 organic peroxide rules. TBHP is consumed as a radical initiator in high-pressure low-density polyethylene production at 200–300 °C and 1,500–3,000 bar, in acrylic emulsion polymerization with reducing agents such as sodium formaldehyde sulfoxylate, and in unsaturated polyester curing with cobalt octoate. The most serious process conflict is contamination of the final TBHP with dissolved iron above 0.1 ppmw; iron promotes radical decomposition even in refrigerated storage, causing pressure buildup and off-spec oxygen content. Terminal products include PVC and polyacrylic dispersions, unsaturated polyester castings, and thermoset countertops.

When TBA Is Converted to tert-Butyl Chloride for Pharmaceutical Alkylation, Moisture and Acid Gas Handling Set the Batch Cycle Time

In a stirred, vented reactor train configured with a caustic scrubber, TBA is contacted with concentrated hydrochloric acid at a molar ratio of 1.2:1 to 1.5:1 HCl:TBA. The reaction is maintained at 25–30 °C; because tert-butyl chloride boils at 51 °C, the exotherm must be removed quickly through an external brine-cooled circulation loop to prevent loss of product to the vent header. The upper organic phase is separated by gravity and washed with 5 wt% sodium bicarbonate, then dried over anhydrous calcium chloride to a water content below 0.02 wt%. The product is used as a tert-butylating agent in the synthesis of hindered amine pharmaceutical intermediates and in the preparation of tert-butyl esters used as carboxylic acid protecting groups. Residual solvent specifications for pharmaceutical intermediates require the tert-butyl chloride to be tested according to USP <467> and ICH Q3C guidelines; the final product must not contain more than 0.5 wt% TBA and 0.01 wt% water. Acid fume generation requires scrubber efficiency above 99% for HCl removal, with pH of the scrubber liquor maintained at 8.0–9.5 by metered sodium hydroxide. Production bottlenecks are associated with the slow phase disengagement at 5–10 °C when the batch is chilled to reduce hydrolysis; therefore, the settler is sized for a linear velocity below 0.3 mm/s. The terminal products are not isolated at the TBA stage but appear as tert-butyl-protected intermediates in cardiovascular and antiviral drug syntheses, where the tert-butyl group is removed later by acid-catalysed deprotection.

Fuel-Grade TBA Co-Solvent Blending in Methanol-Gasoline Systems — Phase Stability and Vapour Pressure Limits

The use of TBA as a co-solvent in methanol-gasoline blends is restricted to terminal blending operations where the water content of the base gasoline is held below 0.01 wt%. At blend ratios of 2.5–5.0 vol% TBA and 5.0–10.0 vol% methanol, phase separation is suppressed at test temperatures down to −20 °C; above these ratios, the water tolerance of the blend falls steeply. Phase stability is measured by the water tolerance test described in ASTM D6422 for ethanol-gasoline blends, adapted for methanol-TBA systems. Vapour pressure must be monitored by ASTM D5191 because TBA addition to gasoline can raise the dry vapour pressure equivalent by 3–7 kPa at 37.8 °C; terminal operators adjust the butane content to maintain the volatility class limits of ASTM D4814. Final blends intended for off-road and racing use are not subject to the same oxygenate mandates as EPA reformulated gasoline; however, the TBA must be derived from a fuel-grade stream with peroxide number below 5 mg/kg to avoid gum formation. The terminal products are limited-volume specialty fuels rather than fungible pipeline gasoline; the practical limitation is the high freezing point of TBA at 25.5 °C and the tendency of methanol-water-TBA ternary mixtures to stratify if exposed to humid air in storage tanks vented to atmosphere. Therefore, storage tanks are blanketed with dry nitrogen and fitted with desiccant breathers. Published vapour-liquid equilibrium data for methanol-TBA-gasoline ternary systems outside 40 °C is limited; terminal blending therefore relies on small-scale phase separation tests under ASTM D6422.

In lyophilization of poorly soluble injectable compounds, TBA is added to the formulation as a crystallizing co-solvent at 5–20 wt% relative to the aqueous bulk. The formulation is filtered through 0.22 µm sterilizing-grade membrane and filled into glass vials before being placed on temperature-controlled shelves. The freezing ramp is set to reach −45 °C at 1.0 °C/min; TBA forms needle-like crystals that leave a porous matrix in the frozen plug, reducing primary drying resistance. Chamber pressure during primary drying is maintained at 50–150 mTorr with shelf temperature at −20 °C to 0 °C; the product temperature is monitored by thermocouples and maintained below the collapse temperature. Because TBA is volatile and residual solvent levels must meet ICH Q3C Class 3 permitted daily exposure of 50 mg/day, the secondary drying phase is extended at 25–40 °C until the cake moisture is below 1.0 wt% and TBA is not detected by headspace GC. The production lyophilizer is equipped with a condenser operating below −70 °C to trap TBA vapor; if the condenser warms above −60 °C, TBA vapours can enter the vacuum pump and contaminate the oil. Terminal products include sterile injectable powders for oncology and anti-infective formulations where the active pharmaceutical ingredient has poor aqueous solubility. The process limitation is the freezing point of TBA at 25.5 °C; bulk TBA lines in the formulation suite must be heat-traced above 30 °C to prevent solidification during metering.