Sulfuric acid alkylation of an isoamylene-rich C5 raffinate is practiced where a steam cracker C5 cut, after butadiene recovery and isoprene extraction, retains sufficient tertiary pentenes to justify conversion into motor-gasoline-range branched paraffins. The feed is not a clean model olefin; it contains 2-methyl-2-butene, 2-methyl-1-butene, 3-methyl-1-butene, n-pentane, isopentane, cyclopentane, minor C6 paraffins, and trace unconjugated diolefins. For a representative raffinate, gas chromatographic analysis by ASTM D2163-20 shows 40–55 wt% total isoamylene isomers, of which 2-methyl-2-butene generally accounts for 35–45 wt% of the total feed, 2-methyl-1-butene 8–15 wt%, and 3-methyl-1-butene 1–5 wt%. The paraffinic diluent fraction, usually 25–40 wt%, lowers the effective olefin concentration in the reactor and changes both the required isobutane recycle and the acid-hydrocarbon dispersion pattern. In commercial operation, batch-to-batch variance of 2-methyl-2-butene of ±5 wt% is commonly observed as upstream steam cracker severity and isoprene extraction cut points change; this variance shifts heat release and acid consumption more than would be expected from total C5 olefin content alone because the tertiary isomer is significantly more reactive than the secondary isomers. The feed may also contain 0.05–0.30 wt% total conjugated diolefins, measured as maleic anhydride reactive material, and 10–30 mg/kg total sulfur downstream of a caustic wash. The paraffinic diluent does not alkylate; its role is primarily as a heat sink and as a vapor-pressure control component in the effluent refrigeration loop, but at high diluent levels it reduces olefin throughput per unit reactor volume and may increase the effective energy consumption per barrel of alkylate.

How Does Acid Strength Drive Branching Selectivity in Isoamylene Alkylation?

The catalytic cycle in sulfuric acid alkylation of isoamylene proceeds through protonation of the tertiary pentene to form a tert-pentyl cation or its rearranged isomer. Hydride transfer from isobutane produces a tert-butyl cation; the tert-butyl cation then reacts with another isoamylene molecule to form a C9 carbocation, which is saturated by a second hydride transfer from isobutane. Sulfuric acid strength controls the steady-state concentration of carbocation intermediates and the relative rates of hydride transfer versus oligomerization. Commercial alkylation is operated at 90–92 wt% H₂SO₄, with the lower boundary at 89 wt% and the upper boundary near 93 wt%. Below 89 wt%, the hydride transfer rate falls more rapidly than olefin protonation, so C10+ acid-soluble oils increase, alkyl sulfate esters survive into the hydrocarbon phase, and the acid phase becomes more viscous. Above 93 wt%, the thermodynamic activity of water is low, but the acid can sulfonate or oxidize the more reactive isoamylene isomers; the yield loss appears as increased sulfur-containing acid-soluble oil and sulfur dioxide evolution in the spent acid draw. Acid strength is not controlled by acid addition alone; water entering with the feed and with recycled isobutane continuously dilutes the acid inventory. Consequently, acid strength is maintained by continuous spent acid withdrawal and fresh acid addition, with the spent acid draw typically set to keep circulating acid at 90–92 wt% and acid-soluble oil content below 6–8 wt%. The titratable acidity of the circulating acid is measured by density against ASTM D4052-19 and by hydroxide titration, with the density of fresh 90–93 wt% acid in the range 1.80–1.84 g/cm³ at 15°C. Spent acid density falls as water and acid-soluble oils accumulate; a circulating acid density below 1.75 g/cm³ is typically interpreted as a process alarm because it corresponds to excessive water pick-up or severe acid degradation.

The isoamylene isomer distribution also influences the proportion of undesirable C10+ material. 2-Methyl-2-butene is the most reactive because it forms a tertiary carbocation with a low barrier; 2-methyl-1-butene can isomerize to 2-methyl-2-butene under acid conditions, while 3-methyl-1-butene requires skeletal rearrangement before alkylation. Published data for the alkylation of pure 2-methyl-2-butene with isobutane show high yield of branched C9 paraffins when the isobutane-to-olefin molar ratio is kept above 8:1 and acid strength remains above 90 wt%. However, C5 raffinate streams contain paraffinic diluents that lower dissolved olefin concentration and increase the apparent viscosity of the hydrocarbon phase; this changes the mass-transfer rate across the acid-hydrocarbon interface and may require a higher acid-to-hydrocarbon dispersion ratio than butylene operation. In a production contactor, the feed is typically mixed with isobutane recycle so that the combined reactor feed contains 50–65 vol% isobutane and 5–12 vol% olefin. The raffinate is not injected directly into the acid inventory without dilution because localized olefin-rich regions promote rapid oligomerization and acid esterification.

If Conjugated Diolefins Enter the Contactor Before Selective Hydrogenation

When the C5 raffinate has not been subjected to selective diolefin hydrogenation, acid consumption and the rate of spent acid production become dominated by polymerization of conjugated dienes rather than by the desired alkylation. A diolefin content of 0.5 wt% or higher, measured by UOP 326-19, can produce an acid-soluble oil build-up exceeding 1.5 lb H₂SO₄/gal alkylate, compared with 0.7–1.0 lb H₂SO₄/gal alkylate for the same feed after diene removal to below 0.30 wt%. The mechanism involves protonation of the conjugated diene to a resonance-stabilized allylic cation, which undergoes successive addition reactions with additional diene or monoolefin molecules before hydride transfer can intercept the chain. The resulting acid-soluble oils contain unsaturated cyclic and polycyclic structures, increase the density and viscosity of the acid phase, and coat the impeller and tube surfaces of the contactor. In a selective hydrogenation unit, the raffinate is treated over a palladium-alumina catalyst at 40–80°C, 1.5–3.0 MPa hydrogen partial pressure, and liquid hourly space velocity 2–6 h⁻¹, converting conjugated dienes to monoolefins while minimizing isoamylene saturation. If hydrogenation is operated too hot or with poor hydrogen distribution, the catalyst hydrogenates tertiary pentenes to isopentane, thereby destroying the alkylation feedstock. The hydrogenated raffinate should contain less than 0.10 wt% isoprene and less than 0.30 wt% total conjugated dienes before entering the sulfuric acid contactor. Trace acetylenes, if present, are also harmful; their removal is typically accomplished in the same selective hydrogenation step because they can form unstable acid-soluble products and increase the sulfate content of the spent acid.

Commercial sulfuric acid alkylation of C5 raffinate is carried out in a horizontal, pressure-retained contactor with an internal circulation tube and an impeller designed to generate a fine acid-hydrocarbon emulsion. In a Stratco-type contactor, the impeller draws the acid-hydrocarbon mixture from the tube and discharges it through a top-mounted header into the reactor zone; the resulting circulation rates are typically 5–10 reactor volumes per minute, and impeller tip speed is maintained in the 20–30 m/s range to produce acid droplets in the 50–250 µm size range. The acid phase is the continuous phase in normal operation, occupying 50–60 vol% of the emulsion, while the hydrocarbon phase is dispersed. Temperature control is achieved by autorefrigeration: the reactor operates at 4–10°C and the corresponding isobutane vapor pressure; vapor is withdrawn from the contactor, compressed, condensed, and returned. The reaction pressure is therefore not fixed independently but is determined by the vapor pressure of the isobutane-rich refrigerant mixture at the desired temperature, typically 50–150 kPa gauge for 4–10°C depending on propane and normal butane content. The isobutane recycle is mixed with the olefinic raffinate upstream of the contactor to give an isobutane-to-olefin molar ratio of 8:1–14:1, with the higher end preferred for isoamylene-rich feed because C5 olefins form C9 carbocations that have a greater tendency to oligomerize before hydride transfer if the local isobutane concentration is low. Olefin liquid hourly space velocity based on acid inventory is typically 0.2–0.5 h⁻¹; higher space velocity operation is possible only with very high isobutane recycle and good heat removal. Published heat of reaction for isobutane alkylation with C3–C5 olefins is approximately 1,200–1,500 kJ/kg of olefin converted; the C5 raffinate heat release is at the upper end because higher molecular weight products raise the exotherm per mole of olefin converted only modestly but the sensible heat demand of the diluent paraffin is significant.

Effluent-Refrigerated Reactor Mechanical Limits and Acid Carryover

In production-scale effluent-refrigerated contactors, the mechanical constraints are set by the impeller drive, the circulation tube erosion rate, and the acid carryover threshold into the settling zone. The emulsion must be stable enough to maintain high interfacial area for mass transfer but not so stable that the hydrocarbon phase cannot separate in the acid settler. Acid carryover into the settler hydrocarbon phase is typically controlled below 25–50 mg/kg total sulfate in the raw alkylate by maintaining acid droplet coalescence and by injecting a water wash. The acid settler residence time is usually 20–40 minutes in the hydrocarbon phase and 20–30 minutes in the acid phase, with the acid phase withdrawn continuously from the settler bottom and returned to the contactor. The impeller is often a two- or three-bladed high-shear device; the drive system must be sized for the viscous acid-hydrocarbon emulsion, which can reach an apparent viscosity of 25–60 cP in the contactor when acid strength falls and acid-soluble oil concentration exceeds 5 wt%. If the spent acid draw is closed for more than 6–12 hours, the acid strength can fall below the operating threshold in a few hours because water in the feed is not removed. The practical lower temperature limit is set by the freezing point of the circulating acid and hydrate phase behavior; sulfuric acid of 90–93 wt% has a freezing point below −10°C, but local cold spots in the refrigeration condenser can cause hydrate or solid deposition if water is present.

Spent acid from C5 raffinate alkylation typically contains 88–90 wt% H₂SO₄, 4–8 wt% acid-soluble oil, and 2–4 wt% water. The acid-soluble oil fraction has a carbon-to-hydrogen ratio higher than the feed and contains sulfur-containing esters; its density is lower than the acid but it remains dissolved or emulsified in the acid phase. The spent acid is transferred to regeneration or neutralization. In many plants, the spent acid is sent to a sulfuric acid regeneration furnace, where the acid-soluble oil acts as fuel and the sulfur values are recovered as SO₂ and converted to fresh acid. The amount of spent acid that must be withdrawn is set not only by acid consumption but also by water accumulation. A water balance around the contactor shows that each 1 kg of water in the feed and recycle stream dilutes 2–3 kg of 98 wt% acid below the target concentration. The feed water specification of 200 mg/kg therefore controls the required acid withdrawal rate. In addition, some water is formed in the reaction of sulfuric acid with oxygenated compounds or by decomposition of monoalkyl sulfates; the exact amount is feed-specific and should be measured by a water balance during the first weeks of commercial operation. Published data for isoamylene-rich raffinate-specific water formation rates are limited, so the design acid withdrawal rate is normally based on butylene alkylation data and then adjusted after an on-site material balance.

Monitoring Water Balance, Spent Acid Viscosity, and Alkylate End Point

Continuous monitoring of the circulating acid and the raw alkylate is required to detect the transition from clean alkylation to acid-soluble oil accumulation. The circulating acid is sampled at the contactor circulation line and analyzed for density by ASTM D4052-19, water by Karl Fischer titration according to ASTM D6304-20, and apparent viscosity by a viscometer method such as ASTM D445-21. The raw alkylate is sampled after the water wash and analyzed for total sulfur by ASTM D6667-14 or ASTM D5453-19a, and for distillation by ASTM D86-20b. The final boiling point of the alkylate is especially informative for C5 alkylation because any oligomerization of isoamylene increases the C10+ fraction and raises the 90% and 95% distillation temperatures. A shift in the 95% point from 160–175°C to above 190°C in a C5-derived alkylate is a direct indication that the hydride transfer rate is insufficient and that C10+ material is accumulating. Similarly, a rise in spent acid viscosity above 15–20 cP at 40°C indicates excessive acid-soluble oil and may require higher acid withdrawal. The research octane number of the stabilized alkylate is measured by ASTM D2699-19; for C5 raffinate alkylate, typical RON values fall in the 90–94 range, but the exact value depends on the C9 isomer distribution and the amount of C10+ material. Motor octane number measured by ASTM D2700-19 is typically 87–91 for the same product. These octane values are lower than those of butylene-derived alkylate because C9 isoparaffins have a lower RON than the C8 isoparaffin 2,2,4-trimethylpentane, but C5 raffinate alkylate still provides a low-sulfur, low-olefin gasoline blending component with favorable vapor pressure.

The operating envelope for sulfuric acid alkylation of isoamylene-rich C5 raffinate is narrower than that for butylene feed because isoamylene is more prone to oligomerization and acid-soluble oil formation. Acid strength should not be allowed to fall below 89 wt% for more than a few hours; the isobutane-to-olefin molar ratio should remain above 8:1; feed diolefin content should be held below 0.30 wt%; and reactor temperature should not exceed 10°C. Feed water should be limited to 200 mg/kg, because water directly dilutes the acid inventory and accelerates formation of a separate spent acid phase. The raffinate should not be combined with amine-based additives or alkaline wash carryover, because the residual base neutralizes sulfuric acid and forms amine sulfates that precipitate in the settler and plug downstream equipment. Oxygenates such as methanol or tert-butanol, if present from upstream gasoline blending operations, should be limited to 50–100 mg/kg in the feed because they readily form sulfate esters and increase acid consumption. If these boundaries cannot be maintained, the unit should be operated in a lower-throughput mode with increased acid withdrawal and increased isobutane recycle until the feed composition returns to specification. Published data for the complete pilot-scale evaluation of this specific raffinate configuration are limited, so the first commercial batch campaign should include a formal mass balance of acid, sulfur, water, and C5 olefin conversion rather than relying solely on butylene alkylation benchmarks.