Narrow Raffinate C4 Feed Envelope Tolerance in Sulfuric and Hydrofluoric Acid Units

Raffinate C4 is the residual butane-butene stream remaining after selective extraction of 1,3-butadiene, isobutylene, or both from mixed C4 cuts generated by fluid catalytic cracking or steam cracking. The term “feed envelope” in alkylation service refers to the multidimensional composition window within which acid consumption, acid strength, product octane, and unit fouling remain within operable limits. For sulfuric acid units the envelope is controlled primarily by the rate of acid dilution and red oil formation. In hydrofluoric acid units the controlling variables shift toward water partitioning, oxygenate-driven organic fluoride formation, and acid-soluble oil polymerization. A narrow raffinate C4 feed envelope occurs when the concentration of conjugated dienes, oxygenates, sulfur species, water, and C3/C5 olefins must be maintained inside tight specification bands to avoid loss of catalyst activity and accelerated corrosion. Published data for this specific configuration is limited for some trace species, but refinery battery-limit transfer records and licensor operating manuals provide the bounds applied in commercial practice. The remainder of this document examines the practical envelope limits in each acid system without reducing the discussion to a single set of fixed specifications, because acid regeneration capacity, metallurgy, and downstream product sulfur requirements differ among manufacturing sites.

What Feed Impurities Dominate Acid Consumption in Sulfuric Acid Alkylation of Raffinate C4?

Sulfuric acid alkylation units process raffinate C4 in a liquid-phase reaction between isobutane and C3–C5 olefins using a circulating acid catalyst at 85–93 wt% H₂SO₄. The principal feed impurities that narrow the envelope are 1,3-butadiene, acetylene derivatives, oxygenates, water, and heavy sulfur species. 1,3-Butadiene entering the reactor undergoes acid-catalyzed polymerization and forms acid-soluble oils, commonly called red oil, which increase acid consumption and viscosity in the acid settler. Refinery experience in Stratco contactor and autorefrigeration units indicates that each 0.1 wt% of 1,3-butadiene in feed can increase acid consumption by roughly 0.2–0.5 lb acid per gallon of alkylate, depending on reactor temperature and isobutane-to-olefin ratio. This range is not universal; published data for this specific configuration is limited, and licensor-specific kinetic models are used for detailed predictions. Water in the raffinate C4 dilutes the circulating acid. A sustained feed water content above 100–200 ppmw lowers the acid strength below the target 89–90 wt% window and increases corrosion of carbon steel equipment. Oxygenates such as methanol, dimethyl ether, and MTBE decompose in the acid phase, generating water and olefin oligomers that further dilute the acid and increase red oil formation. The practical feed envelope therefore requires simultaneous control of 1,3-butadiene below 0.2–0.5 wt%, water below 100–200 ppmw, and total oxygenates below 50–100 ppmw in many refinery specifications. Acid carryover in the downstream fractionator and reboiler fouling are batch-to-batch failure modes observed when the C4 raffinate feed envelope is exceeded for several days. The acid settler interface becomes difficult to maintain when red oil viscosity exceeds the emulsion-breaking capacity of the settler, and the resulting acid-hydrocarbon emulsion can reach the product fractionator, accelerating corrosion in the overhead system.

Hydrofluoric acid units impose a different impurity hierarchy on raffinate C4 than sulfuric acid units because anhydrous hydrogen fluoride is a weaker acid in terms of water tolerance but a more aggressive fluorinating agent for oxygenates and conjugated dienes. Circulating hydrofluoric acid is typically maintained at 85–92 wt% HF with water below 2 wt% and acid-soluble oil below 5 wt%. The feed envelope narrows sharply for oxygenates because methanol, ethanol, and MTBE react with HF to form methyl fluoride, ethyl fluoride, and tert-butyl fluoride, respectively, while liberating water. That water depresses the HF acid strength and increases corrosion of carbon steel and Monel trim. Field data from acid regeneration column reboiler inspections show that elevated oxygenate concentrations correlate with accelerated fouling of the reboiler bundle and deterioration of acid cooler heat transfer coefficients. Sulfur compounds, particularly mercaptans and carbon disulfide, are less acid-consuming than in sulfuric acid service but can degrade product color and generate volatile sulfur compounds in the alkylate. The feed envelope for HF units is therefore most sensitive to water, oxygenates, 1,3-butadiene, and heavy materials that contribute to acid-soluble oil formation.

Water and Oxygenate Partitioning Across the HF Acid Catalyst Phase

Water entering a hydrofluoric acid alkylation unit does not simply dilute the acid; it partitions between the hydrocarbon phase, the HF-water azeotrope, and the acid-soluble oil layer, shifting the acid strength profile across the circulating catalyst loop. In the reactor, water reacts with HF to form a partially ionized acid phase with reduced cracking activity and increased polymerization tendency. The acid regeneration column, operating with a bottoms temperature typically limited to 180 °C to suppress olefin polymerization, strips water and light acid-soluble oils overhead. If the raffinate C4 feed water content exceeds the design water removal capacity, the circulating acid water concentration rises above 2 wt%, and corrosion rates in carbon steel piping and acid cooler channels increase. Oxygenates follow a similar partitioning pathway but with an added complication: the alkyl fluorides produced in the reactor are partially dissolved in the acid phase and partially carried into the acid regeneration column, where they hydrolyze and release water. This internal water generation can exceed the feed water contribution when methanol concentration exceeds approximately 50 ppmw in the C4 feed. Published data for this specific configuration is limited, but the operational response in commercial HF units is to reduce raffinate C4 oxygenate specification to 10–30 ppmw for methanol and 20–50 ppmw for MTBE at the battery limit. The narrow feed envelope for oxygenates in HF units is therefore not solely a reaction chemistry limit; it is also a water balance and corrosion control limit.

Typical refinery battery-limit specifications reported in licensor general guidance and process surveys are summarized in Table 1. Individual designs vary with acid regeneration capacity, metallurgy, and product sulfur specifications, so the values should be treated as representative rather than universal.

Raffinate C4 produced after MTBE or TAME extraction carries residual oxygenates that are not always captured by a simple composition analysis. In sulfuric acid alkylation, MTBE residuals generally hydrolyze to tert-butyl alcohol and methanol in the acid phase. The tert-butyl alcohol then dehydrates to isobutylene, which is a reactive olefin and can enter the alkylation reaction, but the liberated water and methanol remain in the acid phase and reduce acid strength. Refinery experience with MTBE-extracted raffinate C4 shows that a methanol concentration of 50 ppmw combined with 200 ppmw MTBE can produce a measurable acid strength depression of 0.2–0.5 wt% over a 24 h period in a unit without a dedicated acid purge. For HF units the same oxygenates generate organic fluorides that concentrate in the acid regeneration column overhead and may form a separate light fluoride layer that complicates overhead condensing. The feed envelope for raffinate C4 from MTBE units is therefore narrower than for butadiene-extracted raffinate because the oxygenate specification must be verified at the source, not after transfer line mixing with isobutane recycle. Published data for this specific configuration is limited for HF units, but operational troubleshooting reports identify oxygenate breakthrough from the MTBE unit as a recurring cause of acid cooler fouling and off-spec alkylate vapor pressure.

When Butylene Isomer Distribution Narrows the Feed Envelope in HF Units

Butylene isomer distribution in raffinate C4 directly affects alkylate quality and acid consumption, and the effect is more pronounced when the feed envelope is already constrained by oxygenates or dienes. 1-Butene and 2-butene both alkylate isobutane to produce trimethylpentanes, but 1-butene first isomerizes to 2-butene in the acid phase; this isomerization step releases heat and increases the local reactor temperature. In hydrofluoric acid units, a high 1-butene-to-2-butene ratio above 1.0 can reduce the overall octane yield by shifting the product distribution toward dimethylhexanes and heavy ends, especially when the reactor temperature exceeds 35–40 °C. In sulfuric acid units the same shift exists but is partially masked by the higher acid strength and the larger heat sink provided by the circulating acid emulsion. The feed envelope narrows when raffinate C4 contains both a high 1-butene ratio and residual isobutylene above 1–2 wt%, because isobutylene oligomerization competes with alkylation and increases acid-soluble oil formation in HF service. Refinery process engineers use ASTM D2163 compositional data to calculate the butylene isomer ratio and adjust the isobutane recycle rate to maintain the reactor temperature below the threshold. When the 1-butene-to-2-butene ratio exceeds 1.2, the HF unit feed envelope is typically redefined by reducing the C4 feed rate or increasing the isobutane-to-olefin ratio to 12:1–15:1 from the baseline 10:1 to maintain the alkylate RON-MON gap within specification.

Sulfur speciation in raffinate C4 is frequently more important than total sulfur because hydrogen sulfide, carbonyl sulfide, and mercaptans behave differently in the two acid systems. In sulfuric acid alkylation, hydrogen sulfide and mercaptans can be oxidized by the acid to elemental sulfur or sulfur dioxide, which then react to form polysulfides and sulfurous acid species that increase acid consumption and contribute to product color instability. In hydrofluoric acid alkylation, sulfur compounds do not consume HF directly, but they can react with olefins to form organic sulfides and mercaptans that remain in the alkylate and create a distinct odor and possible copper strip test failure. Carbonyl sulfide hydrolyzes in the presence of water to form hydrogen sulfide and carbon dioxide, but the hydrolysis rate is low at typical reactor temperatures. The feed envelope for total sulfur is therefore often set at 10–30 ppmw in both acid systems, but the speciation limit for mercaptans is lower in HF units because of product quality concerns. Metals such as arsenic, antimony, and iron that enter with raffinate C4 or transfer line corrosion products can poison HF catalyst by forming stable metal fluorides and can plug the acid regeneration column trays. Published data for this specific configuration is limited for trace metals, but sampling records from HF acid regeneration columns show iron fluoride deposition on trays when the feed iron content exceeds 0.5 ppmw.

Feed Envelope Verification at Refinery Transfer Lines and Alkylation Unit Battery Limits

At refinery transfer lines and alkylation unit battery limits, feed envelope verification requires analyzers that report composition at intervals shorter than the acid strength response time. The hydrocarbon composition is measured by gas chromatography per ASTM D2163 or ISO 7941, which gives C3–C5 paraffin and olefin distribution including 1,3-butadiene when the column set is configured for diene separation. Diene value by maleic anhydride addition per UOP 326 provides a bulk conjugated diolefin quantification that can be compared against the GC-derived 1,3-butadiene value. Total volatile sulfur is measured by ultraviolet fluorescence per ASTM D6667. Trace oxygenates in C4 are measured by gas chromatography with an oxygen-selective detector or by ASTM D7423 when applicable. Feed water is the most difficult to verify because water can condense in sample lines and cause false readings; sample conditioning with heated vaporizing regulators and PTFE-lined tubing is required. The feed envelope is considered verified when the rolling 24 h average of each critical species remains below the battery-limit specification and no single grab sample exceeds the short-term excursion limit. Operational boundaries include maintaining sample line temperature above 60 °C to avoid water condensation and avoiding copper-containing sample components in HF service to prevent fluoride corrosion. The response lag between feed composition change and acid strength measurement is typically 4–12 h in sulfuric acid units and 2–6 h in HF units, depending on acid inventory and acid regeneration rate. This lag determines the maximum allowable feed rate change when the raffinate C4 source switches between butadiene-extracted and MTBE-extracted streams.