During continuous thermal adduction of high-vinylidene polyisobutylene with maleic anhydride, the coking threshold is governed not by a single bulk reaction temperature but by the coupled response of internal film temperature, anhydride partial pressure, and stagnation residence time at the heat-transfer surface. Process data from forced-circulation reboilers and jacketed stirred reactors operating between 215 °C and 245 °C show that the bulk setpoint can be maintained for 3,000–5,000 hours only when the tube-wall skin temperature remains below 262 °C, the local heat flux stays under 38 kW/m², and the annular flow velocity exceeds 1.7 m/s at the heater outlet. Once the surface temperature exceeds 272 °C for intervals longer than 15 minutes, the deposition rate of carbonaceous foulant increases from a baseline of 0.8–1.2 µm/day to 6–9 µm/day, as inferred from pressure-drop trends and post-run tube inspections. The foulant consists principally of dehydrogenated polyisobutylene backbone fragments, maleic anhydride oligomers, and condensed aromatic nuclei that are detectable by flash pyrolysis gas chromatography–mass spectrometry as a series of alkylbenzene, alkylnaphthalene, and alkylindene fragments. The bulk reaction mixture is typically characterized by a kinematic viscosity at 100 °C of 180–230 mm²/s by ASTM D445 and an acid number after hydrolysis of 0.75–1.15 mg KOH/g by ASTM D664; these values do not themselves indicate incipient coking, but an increase in the 10 mass % distillation bottom Conradson carbon residue measured by ASTM D189 from 0.25 mass % to above 0.40 mass % correlates with the first measurable loss of heat-transfer coefficient in the primary heater. The safe operating envelope is therefore best expressed as a set of simultaneous limits on skin temperature, wall shear stress, free maleic anhydride content, and residual antioxidant capacity rather than as a single allowable reactor outlet temperature.

Because the thermal ene adduction is only moderately exothermic, the dominant heat input is sensible heating of the high-viscosity feed from storage temperature to reaction temperature, and the heating coil operates close to its fouling limit. The feed polyisobutylene with number-average molecular weight between 950 g/mol and 1,300 g/mol exhibits a viscosity of 0.8–1.5 Pa·s at 120 °C, which forces the use of positive-displacement pumps and limits the minimum tube-side velocity that can be achieved without low-flow dead zones. In a typical vertical shell-and-tube heater with 25.4 mm outside-diameter tubes and a tube length of 6.1 m, the transition from laminar to turbulent flow occurs at a Reynolds number of approximately 2,300; below this value, the radial temperature gradient widens, and the near-wall layer can exceed the bulk outlet temperature by 22–28 °C even when the bulk temperature controller remains stable. The resulting thermal boundary layer is the primary locus of coke precursor formation. Control of this boundary requires a minimum wall shear stress of 3.0 Pa and a tube-side velocity of at least 1.7 m/s; below that threshold, deposits grow as a glassy carbon film with a thickness that follows roughly linear kinetics after an induction period. Above 1.9 m/s, the tube-side pressure drop rises sharply, and the risk of erosion-corrosion at return bends becomes measurable, particularly when the process fluid contains traces of solid maleic acid formed by hydrolysis of maleic anhydride at low-temperature shutdown intervals. Consequently, the thermal adduction heater must be operated as a narrow process window with both lower and upper velocity bounds, not merely as a temperature-controlled circuit.

Does Maleic Anhydride Partial Pressure Shift the Adduction/Coking Boundary?

The boundary defined by maleic anhydride partial pressure is asymmetric: increasing the maleic anhydride/polyisobutylene molar ratio from 1.05:1 to 1.30:1 raises the rate of desired succinic anhydride formation but also accelerates the formation of low-molecular-weight maleic anhydride oligomers and fumaric acid derivatives that are sparingly soluble in the reaction mixture. In a closed reactor with a vapour space temperature of 190–210 °C, maleic anhydride partial pressure can rise from 2.8 kPa at a molar ratio of 1.05:1 to 7.5 kPa at 1.30:1, depending on the condenser reflux rate and the inert gas sweep rate. The oligomer fraction is detectable as an increase in the 10 mass % evaporation residue and as a shoulder in the size-exclusion chromatogram at approximately 550–750 g/mol relative to polystyrene standards. When the oligomer concentration exceeds 0.8 mass % of the reactor liquid, the fouling precursor population shifts from high-molecular-weight polyisobutylene degradation products to polar oligomeric species that plate onto cooler metal surfaces in the overhead line and on the reactor wall. The safe upper boundary is therefore not the stoichiometric ratio alone; it is the combination of a maximum maleic anhydride partial pressure of 6.0 kPa, a free maleic anhydride content in the liquid of 0.15 mass %, and a condenser outlet temperature not exceeding 175 °C. Published data for some proprietary inhibitor packages indicates that the coking induction time at 250 °C drops by a factor of 4 when the free maleic anhydride content doubles from 0.10 mass % to 0.20 mass %, although specific plant data for all polyisobutylene molecular-weight distributions is limited. In practice, the condenser and vacuum system must be designed so that maleic anhydride is removed at a rate sufficient to hold the liquid-phase free anhydride below the threshold while avoiding subcooling that can solidify maleic anhydride in vent lines.

In a forced-circulation vertical shell-and-tube reboiler used for heating the polyisobutylene/maleic anhydride reaction mixture, the coking threshold is most directly observed as a divergence between the heat-transfer oil outlet temperature and the process-side bulk temperature. A clean tube bundle operating with a steam or hot-oil supply at 285 °C and a process-side bulk outlet at 238 °C typically yields an overall heat-transfer coefficient of 330–370 W/m²K and a tube-wall temperature estimated at 255–262 °C. After 60 hours of operation, if the heat-transfer coefficient falls below 180 W/m²K, the wall temperature can exceed 280 °C while the bulk temperature remains unchanged; this is the classic self-accelerating fouling threshold. At this stage the pressure drop across the heater rises from a baseline of 0.25 bar to 0.45 bar, and the coke layer on the tube wall has reached an average thickness of 0.15–0.30 mm. The threshold can be detected earlier by monitoring the first derivative of the heater duty at constant process flow; an increase of more than 12% in required hot-oil flow to hold process temperature over a 24-hour interval indicates deposit formation before the bulk temperature error exceeds ±1 °C. A control scheme that trims the hot-oil supply temperature to hold a calculated internal skin temperature below 268 °C is mandatory for extended runs; otherwise, the coke layer can reach 1.0 mm within 120 hours and require mechanical cleaning. The heater should be instrumented with multiple skin thermocouples rather than relying on a single bulk outlet probe, because the final pass and the return bends often run 12–18 °C hotter than the arithmetic average of inlet and outlet process temperatures.

The coking threshold for a given production line varies more with reactor geometry than with the bulk synthesis recipe. The comparative limits in Table 1 are derived from process vessel data and inspection records where the same polyisobutylene feedstock and maleic anhydride ratio were used.

Oxygen ingress at the hot-oil expansion tank or at the reactor seal flush can reduce the induction time for coking by an order of magnitude even when the bulk temperature is held at 225 °C. In a batch reactor with a nitrogen blanket pressure of 0.2 bar g and a leak rate of 0.5 vol %/h oxygen, the concentration of dissolved oxygen in the polyisobutylene feed rises from 10 mg/kg to 45 mg/kg; this produces hydroperoxides that decompose rapidly above 150 °C. The hydroperoxide decomposition products initiate radical chain scission and also promote the homopolymerization of maleic anhydride at the gas–liquid interface. Process audits have identified that the coking rate in a poorly inerted reactor can reach 15–20 µm/day, compared with 1–2 µm/day in a rigorously purged system. The oxygen threshold for safe operation is 20 mg/kg dissolved oxygen in the feed as measured by ASTM D6304 or equivalent; above that limit, the addition of a hindered phenolic antioxidant at 0.10–0.15 mass % is required to preserve the induction period. However, antioxidant addition must be balanced against the tendency of phenolic degradation products to form quinone methide intermediates that can themselves condense into high-boiling carbonaceous residues. The upper limit for phenolic antioxidant in the high-temperature adduction reactor is therefore 0.20 mass %, and the use of amine-based antioxidants is to be avoided because amine degradation products react with maleic anhydride to form dark imides and higher-molecular-weight resins that accelerate fouling. In continuous units, the preferred point of antioxidant injection is upstream of the feed preheater, and the injection rate must be interlocked with the feed oxygen analyzer to prevent over-addition when the feed quality improves.

When High-Molecular-Weight PIB Tail Fractions Enter the Feed

When the polydispersity index of the polyisobutylene feed exceeds 1.8 or when the concentration of species above 2,500 g/mol exceeds 7 mass %, the coking threshold shifts downward by 8–12 °C because high-molecular-weight tails are more susceptible to shear-induced chain scission and thermal dehydrogenation at the boundary layer. In a twin-screw compounder or a continuous static mixer preheater, the high-viscosity tail fraction tends to segregate in low-shear zones behind baffles and at the tube-sheet crossings, creating localized residence times that exceed the bulk average by a factor of 2–3. The degradation of those high-molecular-weight chains produces a sharp increase in the polydispersity of the reactor liquid from a feed value of 1.5 to a product value of 1.9 after 48 hours at 245 °C. The low-molecular-weight fragments generated by scission are relatively volatile; when they condense in the overhead line, they form a sticky wax-like deposit that later hardens into coke. The threshold for this mechanism is best tracked by measuring the mass fraction of pentane insolubles in the reactor liquid according to ASTM D4055; values above 0.05 mass % signal that the high-molecular-weight tail is degrading faster than the stabilizer can scavenge radicals. In such feeds, the safe bulk temperature is lowered from 240 °C to 228 °C and the maximum skin temperature is reduced from 265 °C to 250 °C. Furthermore, the use of high-shear static mixers with an open-area ratio below 65% is not recommended because the narrow gaps generate local wall temperatures that exceed the bulk by more than 35 °C. Feed acceptance testing should therefore include not only number-average molecular weight but also the 10 mass % and 90 mass % cumulative molecular-weight fractions by size-exclusion chromatography, because a narrow specification on average molecular weight alone is insufficient to prevent tail-driven fouling.

Scraped-Surface Thin-Film Adduction Upper Service Temperature

Scraped-surface thin-film evaporators used for removing unreacted maleic anhydride after thermal adduction exhibit a different coking signature from that seen in the primary reactor. The thin-film unit operates at a bulk liquid temperature of 180–205 °C and an absolute pressure of 4–8 kPa, with rotor tip speeds between 3.5 m/s and 5.0 m/s. Under these conditions the wall film is renewed continuously, and the residence time of a liquid element at the hot wall is only 10–30 seconds; this short contact time permits the wall skin temperature to be operated at 215–225 °C without immediate coking. However, when the feed to the thin-film unit contains residual heavy ends at a concentration above 2.5 mass %, the film becomes non-uniform, and the thickness of the boundary layer increases from 0.4 mm to 1.2 mm in the lower third of the rotor. The resulting stagnant patches develop into dark, hard deposits when the wall temperature exceeds 220 °C for more than 8 hours. The threshold for scraped-surface service is therefore controlled by the residual heavy ends rather than by the bulk temperature: a feed with 0.5 mass % heavy ends can be stripped at 210 °C for 800 hours, while the same feed spiked to 2.5 mass % heavy ends has a coking induction time of 60–90 hours at the same wall temperature. The rotor current draw offers a direct indicator of deposit formation; an increase of 10–15% over the clean-tube baseline at constant speed and throughput indicates that the scrapers are no longer removing the full film and that coke is accumulating on the wall. Published data for this specific configuration is limited, but the empirical relationship between heavy-end concentration and scraping efficiency is sufficiently reproducible across multiple rotor geometries to justify a conservative feed limit of 2.0 mass % heavy ends.

Across multiple inspection cycles on continuous adduction plants, the pattern of coke deposition is most severe in the final 10% of the heater tube length and at the return bends, where the flow has passed through the maximum temperature rise and the boundary layer has developed. In a heater with a tube inside diameter of 21.2 mm, a bulk velocity of 1.6 m/s, and an outlet bulk temperature of 238 °C, the measured average coke layer thickness after 2,000 hours was 0.10 mm in the first pass and 0.45 mm in the final pass. The influence of surface roughness is similarly non-uniform: tubes with an arithmetic mean roughness of 0.8 µm show an induction time that is 2.5 times longer than tubes with an arithmetic mean roughness of 3.2 µm when operated at the same bulk and skin temperatures. The surface roughness effect is attributed to the increased availability of nucleation sites for coke precursors and to the greater surface area for adhesion of oligomeric species. Electropolishing of the final pass tubes to an arithmetic mean roughness below 0.4 µm has been reported to extend cleaning intervals by 35–50%, although published data for this exact tube geometry is limited. More reproducible gains are obtained by increasing the tube-side velocity to 1.9 m/s and by replacing the standard 180° return bend with a long-radius fitting that reduces the local pressure-drop coefficient by 20%. In addition, the installation of a strainer upstream of the heater with a mesh opening of 0.5 mm removes particulate precursors that otherwise seed coke formation on the hot wall.

What Limits Liquid Hourly Space Velocity in Continuous Coil Reactors?

The lower liquid hourly space velocity limit in a plug-flow coil reactor is set by the need to maintain sufficient wall shear stress to prevent stagnant boundary-layer residence times. For a coil reactor with a 12.7 mm internal diameter and a length-to-diameter ratio of 600:1, the liquid hourly space velocity of a high-viscosity polyisobutylene/maleic anhydride mixture can be as low as 0.15 h^-1, but only if the tube-side velocity is kept above 1.8 m/s by using a high recirculation ratio. If the liquid hourly space velocity is reduced to 0.10 h^-1 without adjusting the recirculation flow, the near-wall velocity can drop below 0.4 m/s, and the residence time of the thermal boundary layer extends from a few seconds to 30–50 seconds. Under these conditions, the wall film reaches the same temperature as the heating medium and coking occurs within 24 hours. The upper liquid hourly space velocity limit is governed by pressure drop and by the need to achieve the target thermal ene conversion. At a liquid hourly space velocity of 0.50 h^-1, the pressure drop across the coil can exceed 8 bar, and the skin temperature can fall below the required reaction temperature because the heat-transfer coefficient increases but the available heat-transfer area remains fixed. The optimal liquid hourly space velocity for a 1,000 g/mol polyisobutylene feedstock is usually between 0.25 h^-1 and 0.40 h^-1, with a recirculation ratio of 6:1 to 10:1 and a tube-side velocity of 1.9–2.2 m/s. This combination allows a bulk outlet temperature of 235 °C and a skin temperature below 260 °C while maintaining a maleic anhydride conversion of 85–90% per pass. The coil should be divided into at least three independent heating zones so that the heat input can be trimmed in the final pass, where the bulk temperature is highest and the coking margin is smallest.

Routine analytical surveillance for abnormal coking tendency includes the following test method matrix.

If the unit is shut down without first draining the high-viscosity reactor heel, the residual polyisobutylene/maleic anhydride mixture cools below 150 °C and becomes a viscous, highly tacky medium that cements coke particles to the wall. During restart, the heater is typically ramped at 20 °C/h, and the fluid near the wall remains above 260 °C while the bulk temperature is still below 200 °C because of low convective heat transfer. This condition is particularly severe when the tube-side velocity is ramped slowly from 0.2 m/s to 1.8 m/s over 3 hours; the stagnant boundary layer is exposed to high heat flux for an extended period, and coke formation is observed even though the bulk final temperature is within the normal operating window. The recommended restart procedure is to preheat the reactor heel to 120 °C under nitrogen, then increase the hot-oil supply temperature at a rate not exceeding 15 °C/h while maintaining a minimum circulation velocity of 0.6 m/s from the first onset of viscosity reduction. The shutdown procedure should include a light-end flush with a lower-viscosity polyisobutylene fraction of 450–600 g/mol at 150 °C to reduce the residual high-molecular-weight material below 0.5 mass % before cooling. Batch reactors without a circulation pump should be cooled under continued agitation at a wall skin temperature ramp of 10 °C/h or less once the bulk temperature falls below 180 °C, because the thick wall film solidifies preferentially and traps oxidizable heavy ends against the heat-transfer surface.