Production of polymer-grade isobutylene by fixed-bed catalytic dehydration is practiced industrially with tertiary butyl alcohol feedstocks derived from propylene oxide coproduction or from methyl tert-butyl ether cracking. The reaction is carried out over a solid acid catalyst, typically γ-Al₂O₃ or silica-modified alumina, in a fixed-bed configuration at reactor inlet temperatures between 240 °C and 300 °C, with liquid hourly space velocities of 1.0 h⁻¹ to 3.5 h⁻¹ and reactor pressures from 0.15 MPa to 0.55 MPa. Under these conditions, TBA conversion exceeds 99.0 % and isobutylene selectivity exceeds 99.3 % in adiabatic beds with radial temperature deviation below ±5 °C; the principal by-products are diisobutylene, triisobutylene, and trace oxygenates that become controlling variables for downstream polymerization. The fixed-bed arrangement is selected over moving-bed or fluidized-bed configurations for its mechanical simplicity, but its performance is constrained by the endothermic heat demand of the dehydration reaction, by water inhibition on Brønsted acid sites, and by the need to prevent local cold zones that allow unconverted alcohol to accumulate in the condensate system. The heat of reaction for vapor-phase dehydration of TBA to isobutylene and water is reported in the range +30 kJ/mol to +55 kJ/mol, requiring continuous heat input through preheated feed and, in multi-tubular reactors, through shell-side heat transfer fluid.

Feed pretreatment determines whether the fixed-bed catalyst reaches the 99.0 % conversion threshold without excessive coking. Tert-butyl alcohol is typically supplied as a water-saturated azeotrope containing approximately 11.5 wt% water, which must be managed carefully because water adsorbs competitively on the same Lewis and Brønsted acid sites that catalyze dehydration. Preheating is performed in a two-stage exchanger train that first vaporizes the TBA-water mixture at 0.35 MPa and then superheats the vapor to 220 °C before it enters the fixed-bed distributor. The feed filtration system includes a 10 μm guard filter upstream of the preheaters to remove corrosion products and polymeric residues that would otherwise blind the catalyst surface. Process data from commercial TBA dehydration units indicate that a 20 °C reduction in superheat can raise unconverted TBA in the reactor effluent from 0.2 wt% to 2.5 wt%, because the endothermic reaction consumes sensible heat and the resulting temperature drop shifts the equilibrium toward hydration. Consequently, the preheat temperature is held at ±5 °C of the design value by cascading steam control on the second-stage superheater, and the reactor inlet distributor is designed with multiple-vane branch arms to limit radial temperature maldistribution to ±2 °C across the bed cross-section.

What Limits Selectivity When Water Partial Pressure Exceeds 15 Mol%?

The catalytic dehydration of TBA on γ-Al₂O₃ is governed by a Langmuir-Hinshelwood mechanism in which the alcohol dissociates to a surface tert-butoxide and water competes for the same acid sites. Apparent activation energies reported for the temperature window below 300 °C fall between 65 kJ/mol and 85 kJ/mol, while the water adsorption coefficient is typically one to two orders of magnitude larger than the TBA adsorption coefficient at 250 °C. When the water partial pressure at the reactor outlet exceeds 15 mol%, the measured TBA conversion can decline from above 99.0 % to below 97.0 %, and the C8 oligomer yield rises because the remaining TBA and isobutylene participate in secondary condensation on weakly acidic sites. This threshold is not a pure thermodynamic limit but a kinetic suppression point where the surface coverage of active acid sites by water reduces the apparent rate constant sufficiently that the required pseudo-contact time can no longer be achieved in the existing bed length. In adiabatic beds, the outlet temperature drop of 20 °C to 60 °C amplifies the suppression because lower temperatures increase both water adsorption and equilibrium hydration. Consequently, units that operate with high water partial pressure in the feed are often designed with a water-selective pre-distillation step or with a shell-side heat transfer system that maintains outlet temperature above 245 °C.

The tabulated values are representative of γ-Al₂O₃ extrudates with a surface area of 220 m²/g and are not to be used as design values without vendor confirmation. Below 97.0 % conversion, the unconverted TBA partitions into the condensed hydrocarbon phase and must be removed by a water-wash column and a dewatering bed; this imposes an additional energy penalty of approximately 0.4 MJ/kg to 0.8 MJ/kg of isobutylene product compared with operation above 99.0 % conversion.

After condensation against cooling water, the reactor effluent enters a three-phase separator where the vapour fraction containing fixed gases, the liquid hydrocarbon phase, and the aqueous phase are separated. The hydrocarbon phase is routed to a water-wash column operated at 0.5 MPa to remove residual water-soluble oxygenates, followed by a molecular sieve dryer filled with 3A zeolite that reduces water content below 10 mg/kg. The dried C4 stream is then processed in a high-pressure C4 splitter with 80 to 120 theoretical trays and a reflux ratio between 4:1 and 8:1 to remove n-butane, isobutane, and heavier oligomers. The overhead isobutylene product is taken from the top of the splitter, while the bottoms stream containing diisobutylene and triisobutylene is sent to fuel gas or to a heavies cracker. A final guard bed of 13X molecular sieve removes trace oxygenates and residual water after distillation. This sequence is required because polymer-grade isobutylene fed to cationic polymerization must meet simultaneous limits on water, oxygenates, dienes, and sulfur; no single distillation step can achieve the combination of water removal, heavy oligomer rejection, and fixed gas rejection. At ambient relative humidity above 60 %, storage tanks and transfer lines must be blanketed with dry nitrogen and protected with desiccant breathers to prevent water regain after the dryer.

Catalyst Deactivation by Carbonaceous Deposits and Regeneration Limits in Multi-Tubular Beds

Coke deposition on the fixed-bed catalyst proceeds by oligomerization of isobutylene and diisobutylene on strong Brønsted sites, followed by cyclization and dehydrogenation to polycyclic carbon. The coking rate is strongly temperature-dependent; field data from fixed-bed units show that raising the maximum bed temperature above 300 °C can double the carbon accumulation rate within 20 °C. Multi-tubular reactors used in service are typically constructed with tube inner diameters of 25 mm to 50 mm and bed heights of 3 m to 6 m, giving tube length-to-diameter ratios between 60:1 and 240:1. The tubesheets are clad with a wear-resistant alloy, and the shell side circulates a heat transfer fluid such as a triaryl-ether mixture at 260 °C to 285 °C. Catalyst regeneration is performed by controlled air-steam gasification at 450 °C to 500 °C for 24 h to 48 h, with oxygen content ramped to avoid exotherms exceeding 20 °C above the regeneration setpoint. After repeated regenerations, the γ-Al₂O₃ surface area declines from 220 m²/g to below 150 m²/g due to hydrothermal sintering, and the crush strength of the extrudates falls below the minimum specification of 3.0 N/mm; the catalyst load is replaced when pressure drop increases by more than 50 % of the initial value or when conversion at design LHSV falls below 99.0 %.

When Raw TBA Feedstock Contains Residual MTBE Cracking By-Products

When TBA is sourced from an MTBE cracking unit, the feedstock may carry residual methanol, dimethyl ether, C4 olefins, and trace sulfonic acid neutralization products. Methanol dehydrates on γ-Al₂O₃ to dimethyl ether at the same operating temperatures, and concentrations above 500 mg/kg in the feed can generate dimethyl ether concentrations in the product above the 5 mg/kg polymer-grade limit. The fixed-bed catalyst is not able to separate methanol from isobutylene; therefore a pre-distillation column with 40 to 60 theoretical trays is inserted to remove oxygenates before the dehydration reactor. Dimethyl ether has a relative volatility close to isobutylene and must be controlled by feedstock specification rather than downstream distillation. Trace sulfonic acids from MTBE cracking neutralization can deposit on the catalyst surface and reduce the apparent acid site density by 10 % to 30 % over a single 12-month cycle; therefore the feedstock is washed with demineralised water to a conductivity below 2 μS/cm and a pH between 6.5 and 7.5 before vaporization. Published data for the exact long-term performance of alumina catalysts with renewable TBA feedstocks is limited, and pilot evaluation is required when the feedstock contains more than 2 wt% secondary alcohols or aldehydes.

For cationic polymerization of isobutylene, the monomer specification is not a bulk purity value alone but a set of upper limits for substances that act as chain-transfer agents, catalyst poisons, or crosslinking agents. Water above 10 mg/kg consumes Lewis acid initiators such as TiCl₄ or AlCl₃ and creates uncontrolled initiation in living carbocationic systems, leading to broadening of molecular weight distribution beyond the target polydispersity of 1.05 to 1.15. Sulfur compounds above 1 mg/kg poison the initiator and can reduce catalyst efficiency by more than 30 %. Butadiene above 10 mg/kg introduces unsaturation sites that participate in crosslinking and alter the vulcanization kinetics of butyl rubber; commercial butyl rubber requires isoprene unsaturation between 0.5 mol% and 2.5 mol%, and unintended butadiene incorporation shifts the cure profile. Dimethyl ether and methanol above 5 mg/kg are particularly detrimental because they act as chain-transfer agents in carbocationic polymerization and lower the molecular weight of polyisobutylene. For food-contact polyisobutylene and butyl rubber applications, the monomer must also comply with FDA 21 CFR 177.1420 and European Union Regulation (EU) 10/2011 with respect to specific migration limits.

Operational boundaries for fixed-bed dehydration are defined by the need to keep the outlet temperature above the hydration dew point and below the oligomerization acceleration range. Units that process TBA with water content above 12 wt% require pre-drying or increased reactor inlet temperature, but the latter increases coking. Avoid combination of the alumina catalyst with amine-based additives or ammonia injected for corrosion control upstream; amine adsorption on acid sites is essentially irreversible at 250 °C and can reduce conversion by 10 % to 50 % within 24 h. Storage tanks for the finished isobutylene must be pressure-rated to 0.7 MPa, blanketed with dry nitrogen, and protected by relief devices classified under API 520 Part I. Process interlocks should trip the feed preheaters if the reactor inlet temperature drops below 215 °C or if the water content in the feed exceeds 15 wt%. Published data for this specific configuration is sufficient for established TBA routes, but limited for bio-based isobutanol-derived TBA with higher levels of ketones and aldehydes.