Mixed benzene feedstocks derived from catalytic reformate heart-cuts, steam-cracked pyrolysis gasoline after selective hydrogenation, and coke-oven light oil after hydrodesulfurization contain benzene, toluene, ethylbenzene, and mixed xylenes together with C6–C8 non-aromatic hydrocarbons such as methylcyclopentane, cyclohexane, dimethylpentanes, n-heptane, and methylcyclohexane. The separation of BTX aromatics from these non-aromatic hydrocarbons by ordinary fractionation is constrained by close boiling points and azeotropy; cyclohexane and benzene differ in normal boiling point by only 0.6 °C at 101.325 kPa, and methylcyclopentane forms an azeotrope with benzene. Sulfolane extractive distillation introduces tetramethylene sulfone, a polar aprotic solvent with a normal boiling point of approximately 285 °C, a density of approximately 1.26 g/cm³ at 30 °C, and a melting point near 27 °C, into the upper section of a countercurrent column. The solvent preferentially retains aromatic hydrocarbons in the liquid phase and raises the relative volatility of non-aromatic hydrocarbons, allowing a non-aromatic raffinate to leave overhead while a sulfolane–aromatic bottoms stream is routed to solvent recovery. Commercial operations commonly specify solvent-to-feed mass ratios of 2.5:1 to 5.0:1 depending on the non-aromatic content of the feed; circulating solvent is maintained with 0.5–2.0 wt% water to reduce solvent viscosity, lower operating temperatures, and improve selectivity. Units processing reformate-derived mixed benzene with 15–40 wt% non-aromatics typically operate the extractive distillation column at an overhead pressure of 1.2–2.5 bar absolute, a top temperature of 80–120 °C, and a bottom temperature of 150–175 °C. Field experience from commercial fixed-valve tray columns indicates that sudden water ingress or loss of solvent circulation produces foaming in the extractive distillation column, with raffinate carryover and temporary reduction of aromatic recovery; corrective action includes draining free water from the overhead accumulator and restoring solvent circulation before increasing reboiler duty.

How Do Solvent-to-Feed Ratio and Water Content Shift Relative Volatility in Sulfolane Extractive Distillation?

Published vapour–liquid equilibrium data for sulfolane–benzene–cyclohexane systems demonstrate that the relative volatility of cyclohexane to benzene increases from near 1.0 in solvent-free distillation to approximately 2.0–3.0 when the liquid phase contains 70–85 wt% sulfolane. The solvent-to-feed ratio controls the concentration of sulfolane on the trays and therefore the magnitude of the selectivity enhancement. Operation at a low solvent-to-feed ratio reduces the selectivity improvement and forces a higher reflux ratio or a taller column to meet benzene recovery targets; operation at an excessively high solvent-to-feed ratio increases reboiler and condenser duties, raises column pressure drop, and may exceed downcomer liquid-handling capacity. Water is a critical secondary solvent component. Increasing circulating water content from 0.5 wt% to 2.0 wt% raises the polarity of the solvent phase and typically improves the relative volatility of paraffins and naphthenes over benzene, but simultaneously reduces the distribution coefficient of benzene into the solvent-rich phase and therefore reduces solvent capacity. Published data for this specific configuration is limited because phase equilibrium behaviour is highly dependent on the particular C6–C8 non-aromatic distribution; however, commercial licensor guidelines treat water content as a manipulated variable that must be balanced against solvent circulation rate. Excess water above approximately 3.0 wt% can promote secondary liquid–liquid phase separation, increase overhead water accumulation, and destabilize tray hydraulics. The practical control strategy is to maintain solvent-to-feed ratio by flow-ratio control and to manage water content by adjusting the water draw from the overhead accumulator and the solvent regeneration unit.

In the solvent recovery column, the sulfolane–aromatic bottoms stream from the extractive distillation column is stripped under vacuum to recover BTX overhead and regenerate lean solvent for return to the extractive distillation column. The solvent recovery column is operated at a top pressure of 0.25–0.45 bar absolute because the boiling point of sulfolane is approximately 285 °C at atmospheric pressure, and reduced pressure permits recovery of xylenes and ethylbenzene without exceeding thermal stability limits. The bottom temperature is generally held between 160 °C and 180 °C. These temperatures are below the atmospheric boiling point of sulfolane but are sufficiently high to drive residual aromatics overhead; a small amount of stripping steam or an additional reboiler is sometimes used to improve aromatic stripping effectiveness under vacuum. The recovered mixed BTX stream leaving the solvent recovery column overhead is condensed and sent to a water separator, while the lean solvent bottom is cooled and returned to the extractive distillation column. Thermal degradation of sulfolane becomes measurable as temperature approaches 220 °C; therefore, the most serious risk in the solvent recovery column is film temperature in the reboiler rather than the bulk bottom temperature.

When Oxygen Ingress Accelerates Sulfolane Degradation to Sulfur Dioxide and Polymeric Acids

Within the circulating solvent loop, oxygen ingress occurs most frequently through open solvent storage tanks, leaking pump seals on vacuum service, incomplete inert-gas blanketing, and dissolved oxygen in feed or wash water. In the presence of oxygen, sulfolane degradation at solvent recovery temperatures produces sulfur dioxide, acidic intermediates, and dark polymeric residues that increase solvent viscosity and reduce heat-transfer efficiency. Sulfur dioxide can hydrolyse to acidic species that lower solvent pH from the typical operating range of 6.0–8.0 to below 4.0 if left uncontrolled; acidic solvent increases corrosion rates in carbon steel and accelerates further sulfolane decomposition by acid-catalysed ring-opening reactions. Commercial units mitigate this degradation path by maintaining a continuous inert-gas blanket on solvent storage and surge vessels, limiting dissolved oxygen to below 1 mg/kg in circulating solvent, and continuously withdrawing a slipstream of solvent for regeneration. The regeneration step, often a vacuum reclaiming unit or ion-exchange system, removes acidic degradation products and high-boiling polymers. Some units inject a low concentration of an acid scavenger or maintain a slightly buffered solvent condition; however, the circulating solvent system is incompatible with strong oxidizers, and free water pockets in solvent storage should be avoided because water-rich regions can promote hydrolysis and local pH excursions. Published data for this specific configuration is limited because degradation rates depend on temperature history, oxygen partial pressure, and trace metal contamination.

For raw pyrolysis gasoline, feed pre-treatment is often the controlling factor for extractive distillation performance. Raw pyrolysis gasoline contains diolefins and olefins that can polymerise in the extractive distillation column and solvent recovery column, forming fouling deposits on trays and reboiler tubes. Hydrotreating of pyrolysis gasoline to saturate diolefins and reduce olefin content is therefore applied upstream of sulfolane extraction. In contrast, reformate-derived mixed benzene is already low in olefins after catalytic reforming, but may contain trace chlorides, oxygenates, or heavy C9+ aromatics. A prefractionation column is used to control the endpoint of the mixed benzene feed; heavy C9+ aromatics are rejected to avoid accumulation in the circulating solvent and to prevent raising the solvent recovery column bottom temperature. Typical practice limits the feed endpoint to below 150 °C at atmospheric pressure. If olefinic or diolefinic species are present above approximately 0.5 wt%, fouling of the extractive distillation column and reboiler accelerates, and solvent consumption increases due to polymer formation. Clay treating of the mixed benzene feed is also used in some plants to remove trace olefins and polar impurities before extractive distillation.

Managing Solvent Viscosity and Tray Hydraulics in High-Circulation Extractive Distillation Columns

Because sulfolane solidifies below approximately 27 °C, heat tracing of solvent lines and instrument leads is required for shutdown and winter operation, and the solvent circulation system must be designed to handle viscosity excursions during start-up. Sulfolane viscosity is approximately 10 mPa·s at 30 °C, but at extractive distillation operating temperatures of 100–175 °C the viscosity falls below 2 mPa·s, which is sufficiently low for tray operation. Commercial extractive distillation columns for mixed benzene are generally constructed with 60–80 valve trays or an equivalent height of structured packing. The column internals must accommodate high liquid loads because the circulating solvent flow may be three to five times the fresh feed mass flow. Downcomer clear-liquid velocity, tray deck pressure drop, and downcomer backup are therefore more restrictive than vapour hydraulic capacity in many designs. At a solvent-to-feed mass ratio of 4.0:1, a unit processing 50 t/h of mixed benzene circulates approximately 200 t/h of lean solvent through the extractive distillation column; this liquid load dominates the design and operation of the downcomers. Process engineers typically specify a tray spacing of 500–600 mm and use antifoam injection only when needed, because excessive antifoam can contaminate the raffinate and downstream gasoline blending.

Heat integration between the extractive distillation column and the solvent recovery column is a central determinant of utility consumption. The extractive distillation column reboiler uses low-pressure steam or hot oil and must be sized for both the hydrocarbon feed and the circulating solvent. The solvent recovery column reboiler is a high-temperature service because the bottom temperature approaches 180 °C; medium-pressure steam or a hot-oil system is commonly required. The hot lean solvent returning from the solvent recovery column can be used to preheat the mixed benzene feed or to heat the extractive distillation column bottom through cross-exchangers, reducing overall energy demand. The overhead condenser of the extractive distillation column must handle non-aromatic raffinate, water, and a small amount of sulfolane; free water is drawn continuously from the overhead accumulator to prevent water accumulation and to control circulating solvent water content. The vacuum system for the solvent recovery column is typically a liquid-ring vacuum pump or steam ejector with a precondenser; the selection depends on the quantity of stripping steam and the desired top pressure. Inefficient vacuum or rising top pressure in the solvent recovery column forces a higher bottom temperature to maintain aromatic recovery, increasing the rate of sulfolane degradation.

To Prevent Raffinate Off-Specification, Control Overhead Water Accumulation and Reflux Ratio

Raffinate off-specification in sulfolane extractive distillation generally refers to excessive aromatic carryover into the non-aromatic overhead stream. Aromatic losses occur through insufficient solvent circulation, low water content, reduced reflux ratio, or entrainment caused by foaming. The overhead reflux ratio is normally set between 1.0 and 3.0 on a mass basis relative to overhead product, depending on feed non-aromatic content and solvent-to-feed ratio. Too low a reflux ratio reduces the stripping of aromatics from the upper section, while too high a reflux ratio increases condenser duty and may lower the column temperature profile, altering the solvent selectivity. Water accumulation in the overhead accumulator is a common but under-recognized disturbance: if free water is not drawn, it can be recycled as reflux and create unstable tray hydraulics, rapid density changes, and foaming. Operators monitor the overhead accumulator interface level and the circulating solvent water content by Karl Fischer titration, using ASTM D6304 or equivalent, to maintain water in the 0.5–2.0 wt% window. Aromatic carryover into the raffinate above 0.1 wt% is considered abnormal in most integrated BTX recovery units and is corrected by increasing solvent-to-feed ratio, lowering free water entrainment, or adding antifoam after confirming the absence of solvent degradation products.

Once the mixed BTX stream leaves the solvent recovery overhead drum, it is fractionated into benzene, toluene, and xylene products using conventional distillation. The mixed BTX stream contains residual non-aromatics at low concentration, typically below 0.1 wt%, and may contain trace sulfur or nitrogen compounds if the feed was not adequately hydrotreated. A benzene column operates at atmospheric or slightly elevated pressure to produce a benzene product meeting ASTM D2359 with a minimum purity of 99.9 wt% and a maximum non-aromatic content of 0.05 wt%. The toluene column produces nitration-grade toluene meeting ASTM D841 with a minimum purity of 99.8 wt%, and the xylene column produces mixed xylenes meeting ASTM D5211 with a minimum aromatic content of 99.5 wt%. Trace impurities in product BTX streams are measured by gas chromatography using ASTM D7504, which provides a consistent method for benzene, toluene, ethylbenzene, and xylene distribution. The raffinate overhead from the extractive distillation column is sent to gasoline blending or to a naphtha hydrotreater, depending on its sulfur and olefin content. Energy integration across the BTX fractionation train and the extractive distillation unit is normally optimized at the owner-operator level, and published data for this specific integration is limited.

Where solvent thermal stability intersects hydraulic capacity, operational boundaries are defined by the reboiler metal temperature, the circulating solvent quality, and the feed endpoint. The extractive distillation column should not be operated above a bottom temperature of 175 °C for prolonged periods because sulfolane degradation increases rapidly as the local film temperature approaches 220 °C. The solvent recovery column reboiler must be designed with a tube-wall temperature margin; high heat flux can produce local film temperatures above the bulk bottom temperature even when the bulk temperature is within the normal range. The solvent system is incompatible with strong oxidizers, and oxygen partial pressure in storage should be maintained below 0.5 kPa by inert-gas blanketing. Free water pockets should not be allowed to remain in solvent storage vessels because water-rich regions under oxygen ingress can hydrolyse sulfolane and generate acidic species. Circulating solvent pH should be held between 6.0 and 8.0; excursions below 5.0 require immediate slipstream regeneration or pH adjustment. Heavy C9+ aromatics must be rejected from the feed because their accumulation raises solvent recovery column bottom temperature and reduces solvent selectivity. The solvent recovery column vacuum system should be maintained to keep top pressure below 0.45 bar absolute; loss of vacuum forces a temperature increase that shortens solvent life and raises acid number.