Feed purity specifications for 1-butene entering secondary butanol dehydrogenation units are frequently defined at the battery limits of the C₄ hydration front-end and again at the secondary butanol loading pump. In integrated operations, the 1-butene cut is generated from a steam cracker C₄ stream after extraction of 1,3-butadiene, selective hydrogenation of residual acetylenes and dienes, and removal of isobutene by methyl tert-butyl ether synthesis or isomerization to 2-butene. The resulting 1-butene-enriched raffinate is water-washed, dried over molecular sieve or alumina, and fed to an acidic ion-exchange resin hydration reactor at 5.0 to 7.0 MPa and 140 to 170 °C. Per-pass olefin conversion in this equilibrium-limited hydration is generally below 10 to 12 mol%, so the unreacted C₄ raffinate is recovered, the aqueous secondary butanol product is sent to concentration, and a large C₄ recycle stream is returned to the reactor. Because the recycle flow is typically 8 to 12 times the fresh 1-butene feed flow, any impurity with a volatility between butane and 1-butene or with a tendency to oligomerize on the acidic resin is amplified in the recycle loop. The 1-butene feed purity limit is therefore set not only by the dehydrogenation catalyst but by the hydraulic, thermal, and fouling limits of the hydration and recycle sections. Published data for the exact limits of every licensed configuration is limited; however, the general technical logic is supported by standard equilibrium data and operating experience with C₄ hydration processes.

When the 1-Butene Feed Purity Falls Below 95 mol%

At a battery-limit specification of 95.0 mol% 1-butene, the remaining 5 mol% is typically distributed among butane, 2-butene, isobutene, residual 1,3-butadiene, and heavier C₅₊ olefins. The operational consequences of falling below this level are non-linear. A 1-butene concentration below 95 mol% increases the fraction of 2-butene and butane that must be recycled through the hydration reactor, because 2-butene hydration to secondary butanol is significantly slower than 1-butene hydration over the same sulfonic acid resin. The recycle loop vaporization load increases, and the hydraulic pressure drop across the resin bed rises. In addition, isobutene hydration produces tertiary butanol, which cannot dehydrogenate to methyl ethyl ketone and which can back-dehydrate to isobutene in the secondary butanol concentration column, creating a light alcohol recycle that consumes energy and lowers the yield of methyl ethyl ketone. The most damaging impurity is residual butadiene. At resin operating temperatures, butadiene participates in acid-catalyzed oligomerization and forms high-molecular-weight fouling deposits that increase pressure drop and block flow distribution channels. For a fixed-tube hydration reactor with 28 mm internal diameter tubes and 3.0 m tube length, a pressure drop increase from 150 kPa to 400 kPa across the catalyst bed may indicate partial flow maldistribution and can require an early catalyst change. The 1-butene specification of 95.0 mol% is not a thermodynamic threshold but a practical boundary determined by the recycle concentration factor and the catalyst cleaning interval. Published data for the precise relationship between feed purity and cycle length in this specific configuration is limited, but licensor operating manuals typically tie sodium, sulfur, and diene levels to shorter cycle times when the C₄ feed is not pre-treated.

Impurity accumulation in a closed C₄ recycle loop is governed by relative volatility, purge flow, and chemical conversion. Light ends such as propane and propylene are purged through the vent condenser; heavy ends such as C₅ olefins are removed in the debutanizer bottoms. The difficult impurities are those with volatility close to 1-butene: 2-butene, isobutene, and residual butadiene. These species accumulate unless selective catalysts or reactive separation are used. In a typical C₄ purification train, the 1-butene column is operated at 0.6 to 1.2 MPa, with reflux ratios of 15 to 25, requiring 150 to 250 theoretical stages because the relative volatility between 1-butene and isobutene is only about 1.15 at 0.8 MPa. Feed purity therefore becomes a capital-intensity problem as much as a catalyst lifetime problem. If the column cannot achieve the required split, the 1-butene cut will carry isobutene into the hydration reactor. The acid resin will produce tertiary butanol and will also oligomerize isobutene, leading to resin fouling and short run lengths. In addition, residual butadiene will undergo acid-catalyzed dimerization and polymerization within the recycle loop, accumulating as heavy material and eventually reaching the resin surface. The result is a progressive increase in pressure drop, a decline in hydration conversion, and periodic washing or regeneration of the resin bed. These effects are measurable as shifts in the aqueous secondary butanol concentration from a normal operating band of 8 to 15 wt% down to 5 to 8 wt%, requiring higher recycle rates and increased reboiler steam consumption.

What Impurities Provoke Irreversible Dehydrogenation Catalyst Damage?

The dehydrogenation section typically uses a copper-zinc-alumina or copper chromite catalyst in a fixed-bed adiabatic or tubular reactor at 230 to 280 °C and near atmospheric pressure. Secondary butanol is vaporized, mixed with steam or nitrogen diluent, and passed over the catalyst at an LHSV of 0.5 to 1.5 h⁻¹; conversion is equilibrium-limited and normally lies between 70 and 90 mol% per pass. Sulfur compounds are the most serious irreversible poisons for these copper-containing catalysts. Hydrogen sulfide, carbonyl sulfide, and low-molecular-weight mercaptans can be present in the C₄ feed as a result of cracker gas desulfurization carryover or from sulfur-containing liquefied petroleum gas components. If these sulfur compounds are not removed, they eventually report to the secondary butanol product or to the recycle C₄, depending on their partition coefficients. In the dehydrogenation reactor, sulfur bonds strongly to copper sites as cuprous sulfide, destroying the metallic copper surface responsible for alcohol dehydrogenation and shifting the selectivity toward dehydration products such as n-butenes. A sulfur concentration of 2 mg/kg in the secondary butanol feed is often specified as the upper limit for long catalyst life, and a concentration above 5 mg/kg can reduce the expected catalyst life from several years to less than one year. Chlorides are similarly damaging; organic chlorides from caustic tower carryover or from extraction solvents can hydrolyze to hydrochloric acid in the presence of hot water and cause copper chloride species to form and migrate, leading to catalyst sintering and reactor corrosion. A chloride limit of 1 mg/kg in the final secondary butanol feed is therefore applied. Total sulfur in the 1-butene feed is controlled to 5 mg/kg maximum and is determined by ultraviolet fluorescence according to ASTM D6667-14 after vaporization of the liquid sample. The presence of acetaldehyde or other carbonyls in the feed is also restricted because they can undergo aldol condensation on basic sites or on reactor walls and form coke precursors; a carbonyl limit of 50 mg/kg as acetaldehyde is a representative industrial target. Published data for the exact sulfur poisoning rate of each copper-based catalyst is limited, but the non-selective chemistry is well established.

Sampling and analytical reproducibility are as important as the numerical limits. A C₄ stream containing more than 10 mg/kg water or sulfur compounds is analyzed using sample cylinders with internal pressure maintained above vapor pressure, typically following ASTM D1265-05 for manual sampling of liquefied petroleum gases. If the sample is partially flashed before injection, the measured composition will overestimate light ends and underestimate water and sulfur species, because heavier and polar components remain in the liquid phase and may not be homogeneously volatilized. For this reason, the feed analysis system must include heated vaporization at the sample point, passivated stainless steel transfer lines, and a backflushed gas chromatography column to separate oxygenates, sulfur compounds, and heavy C₅₊ material without masking the 1-butene peak. The precision of ASTM D2163-14 under repeatability conditions is generally sufficient for feed purity decisions, but the reproducibility between off-line laboratory samples and continuous process analyzer readings can be widened by sample transport time, ambient temperature changes, and the presence of trace oxygen. Oxygen in the sample cylinder can initiate peroxidation of butadiene if the feed is not stabilised with an inhibitor; this reaction can produce low-chlorine or high-carbonyl species that are not present in the actual feed but appear in the analysis. Continuous online analyzers using gas chromatography with thermal conductivity or flame ionization detection are therefore preferred for controlling the 1-butene column, while laboratory methods are retained for referee testing and for sulfur, water, and chloride speciation.

Vapour-Liquid Equilibrium Pinch Points in C₄ Purification Columns

The removal of isobutene and butadiene to the levels required by the hydration and dehydrogenation sections is often accomplished by superfractionation or extractive distillation. In a simple distillation column operating at 0.6 to 1.2 MPa, the relative volatility of 1-butene relative to isobutene is only 1.10 to 1.20. A column split between isobutene and 1-butene requires 150 to 250 theoretical stages and reflux ratios of 15 to 25. This structural requirement explains why a large fraction of the capital cost of a secondary butanol plant is in C₄ purification. If the feed is not sufficiently separated, the 1-butene cut will carry isobutene into the hydration reactor; the acid resin will produce tertiary butanol and will also oligomerize isobutene, leading to resin fouling and short run lengths. Some units instead use a methyl tert-butyl ether reactor to selectively convert isobutene to methyl tert-butyl ether, allowing subsequent distillation to remove the heavier ether. This approach can reduce isobutene in the 1-butene feed to below 0.1 mol% but introduces methanol into the C₄ raffinate; methanol must then be water-washed from the C₄ stream to avoid oxygenate contamination of the hydration reactor and the downstream secondary butanol. The water wash column is typically operated at 1.5 to 2.0 MPa with countercurrent water at a water-to-feed mass ratio of 0.3 to 0.6. Published data for the specific column stage count of a particular licensor is limited, but the volatility constraints are fixed by standard C₄ vapour-liquid equilibrium data.

After hydration, the aqueous secondary butanol solution from the resin reactor contains 8 to 15 wt% secondary butanol, with trace ethers, dissolved C₄, and resin leachates. The concentration column is operated under slight pressure to avoid C₄ vaporization and to allow the wet alcohol to be stripped from dissolved C₄. Secondary butanol and water form a minimum-boiling azeotrope at approximately 88.5 °C at 101.3 kPa; this azeotrope is fed directly to the dehydrogenation reactor without complete drying. A water content of 0.5 to 1.5 wt% in the vaporized secondary butanol is generally acceptable because steam acts as a diluent that shifts the equilibrium conversion by reducing the partial pressure of hydrogen and inhibiting catalyst coking. However, if the 1-butene feed contains excess water or if the C₄ dryers are not in service, the aqueous stream becomes overloaded, reboiler steam consumption increases, and the water concentration in the dehydrogenation feed may exceed 2 wt%. At these higher water levels, the equilibrium conversion of secondary butanol drops and the downstream methyl ethyl ketone-water azeotrope separation load increases. Vapour-liquid equilibrium constraints in the methyl ethyl ketone recovery train therefore connect the front-end 1-butene feed water limit of 10 mg/kg to the back-end distillation column reboiler duty. In this way, feed purity is not a single analytical value but a continuous operating boundary that links C₄ purification, hydration resin condition, dehydrogenation catalyst selectivity, and methyl ethyl ketone distillation hydraulics.