Salable naphthalene yield from C10+ cracked cuts is governed primarily by the location of the crystallisation setpoint relative to the metastable zone boundary of the feed and by the temperature distribution across the cooling surface. Feedstocks obtained from steam cracker quench oil distillation or heavy aromatic pyrolysis gasoline after hydrotreating typically carry naphthalene from 12 wt% to 28 wt%, alpha-methylnaphthalene from 8 wt% to 14 wt%, beta-methylnaphthalene from 5 wt% to 10 wt%, biphenyl from 2 wt% to 5 wt%, acenaphthene from 0.5 wt% to 2.0 wt%, and indene from 1 wt% to 3 wt%, with the balance composed of higher alkyl naphthalenes, partially hydrogenated polycyclic hydrocarbons, and residual light tar. Pure naphthalene exhibits a melting point of 80.3 °C and an enthalpy of fusion of 19.1 kJ/mol. In this multicomponent matrix, the crystallisation point is not adequately described by an ideal eutectic calculation because close-boiling polycyclic aromatics participate in solid solution formation, and the apparent liquidus temperature determined by cooling exotherm onset under ASTM E794-06(2018) can range from 52 °C to 68 °C depending on naphthalene concentration, methylnaphthalene distribution, and moisture content. The product sold as naphthalene generally must meet a melting point of 79.5 °C to 80.1 °C, corresponding to 98.5 wt% to 99.5 wt% naphthalene by gas chromatography. In a forced-circulation scraped surface crystalliser, a wall-to-slurry temperature difference of 8 K to 12 K generates local surface temperatures below the metastable zone, causing heterogeneous nucleation rather than controlled growth. The control objective is to maintain the bulk slurry temperature within ±1.5 °C of the designated setpoint and to avoid local cold spots on the coolant side. Temperature measurement is performed with Pt100 resistance thermometers meeting class A accuracy of ±0.15 °C at 0 °C under DIN EN 60751:2009, installed in a thermowell exposed to the slurry rather than on the shell side. Without this precision, excursions of 2 °C to 3 °C below the intended crystallisation point are sufficient to depress salable naphthalene recovery by 4 to 7 absolute percentage points because occluded methylnaphthalenes are incorporated into the crystal lattice and cannot be removed by routine washing.

What Limits Cooling Ramp Rate in Suspension Crystallisers Processing C10+ Resid Fractions?

In suspension crystallisation of C10+ cracked cuts, the cooling ramp rate is limited not by the available heat transfer capacity of the scraped surface exchanger but by nucleation kinetics and the risk of exceeding the metastable zone width. A pilot unit with a 0.6 m diameter, 6.0 m² heat transfer surface vertical scraped surface crystalliser, equipped with scraper blades rotating at 120 rpm and jacket coolant supplied from a secondary refrigerant loop at −5 °C to 5 °C, can maintain bulk slurry cooling ramps of 2 K/h to 5 K/h when seed crystals of 200 µm median diameter are injected at 0.5 wt% of the feed. Faster ramps above 8 K/h produce bimodal crystal size distributions with a fines fraction below 50 µm, which increases filter cloth blinding and reduces the filtration rate from 250 kg/m²·h to below 80 kg/m²·h on a rotary vacuum belt filter using a 10 µm polypropylene filter cloth at 0.4 bar vacuum differential. The metastable zone width measured by controlled cooling with focused beam reflectance measurement in a 2 L laboratory-scale crystalliser has been reported at 4 K to 7 K for a feed containing 22 wt% naphthalene. Operating within this zone with tight coolant control allows growth of larger crystals up to 600–800 µm d50, but local wall temperatures below the zone boundary generate a layer of polycrystalline scale that reduces heat transfer by 20–30% within 48 h of continuous operation. Published data for this specific configuration is limited, but observations from analogous para-xylene suspension crystallisers indicate that shear-induced attrition and staged seed addition are more effective than high coolant differentials for maintaining crystal size.

Thermal Profiling of C10+ Cracked Cuts Using Discontinuous Peritectic Step Analysis

Thermal profiling of C10+ cracked cuts using discontinuous peritectic step analysis provides the liquidus and metastable zone limits that define the allowable crystallisation temperature range. A discontinuous peritectic step analysis is performed by equilibrating a filtered feed sample at 80 °C, programming a cooling ramp of 0.5 K/min in a differential scanning calorimeter according to ASTM E794-06(2018), and recording the onset of the first exothermic crystallisation event. The onset temperature for a feed with 20 wt% naphthalene is typically observed between 58 °C and 62 °C, while the minimum crystallisation temperature for the residual oil is 8 K to 15 K lower depending on the mixed methylnaphthalene content. The thermal profile also identifies a secondary exotherm assigned to co-crystallisation of biphenyl and indene from 38 °C to 44 °C, which must be avoided because those impurities can form a sticky interfacial layer between naphthalene crystals and the wash solvent. In continuous operation, the crystalliser setpoint is placed 3 K to 5 K above the secondary exotherm and at least 2 K below the primary onset to balance recovery against impurity inclusion. A three-stage crystallisation sequence using stepwise setpoints of 62 °C, 55 °C, and 48 °C on a falling-film dynamic crystalliser has been used to produce naphthalene with a final melting point of 79.7 °C from a 18 wt% naphthalene feed. The thermal profile for this specific configuration should be confirmed on each new feed batch because changes in cracker severity shift the ratio of alpha- to beta-methylnaphthalene, and the solid-liquid phase boundary is sensitive to that ratio. When the alpha-methylnaphthalene content exceeds 12 wt%, the primary crystallisation onset can fall by 3 K, requiring a downward adjustment of the first-stage setpoint to maintain the same naphthalene recovery.

Slurry temperature control in the crystalliser vessel is achieved with split-range control of coolant flow and residence time, not by varying the cooling medium temperature alone. In a production-scale suspension crystalliser processing 20 t/h of C10+ heavy aromatic oil, the slurry temperature is regulated by a cascade control strategy in which the primary controller receives the Pt100 signal from the crystalliser outlet and the secondary controller manipulates the jacket coolant flow through a split-range valve pair. The primary setpoint is usually 55 °C for a feed containing 20 wt% naphthalene, with a deadband of ±0.5 °C and an output limit of 45–65 °C. The secondary coolant return temperature is constrained to 45–55 °C so that the inside wall temperature never falls below 45 °C. A feed temperature lower than the coolant return setpoint causes rapid scaling on the tube side; the heat transfer coefficient measured by drained vessel testing can decrease from 180 W/m²·K to 90 W/m²·K within 24 h. To recover salable naphthalene, operators use a scheduled warm-up cycle at 70 °C for 60 min to melt crystalline scale before returning to normal operation. The yield loss during such a warm-up cycle is approximately 3–5% of the normalised naphthalene inventory because crystals are dissolved and must be re-formed under controlled conditions. This dynamic constraint is one reason why naphthalene crystallisation operations require intermediate storage capacity equivalent to 12 h of feed flow and a clarified mother liquor return line to recover dissolved naphthalene.

Scraped-Surface Heat Exchanger Fouling and Naphthalene Crystal Habit Modification

Scraped-surface heat exchangers employed for naphthalene crystallisation from C10+ cracked cuts are especially vulnerable to fouling when the internal scraper tip speed falls below 1.5 m/s or when the coolant enters below the crystallisation onset of the concentrated boundary layer. The boundary layer at the tube wall concentrates naphthalene to saturation before the bulk slurry reaches the same temperature, producing a radial composition gradient. If the wall-to-bulk differential is maintained at 5 K or less and the scraper tip speed is kept between 2.0 m/s and 3.5 m/s, crystal habit remains predominantly plate-like with a mean aspect ratio below 2:1. When the differential exceeds 10 K, the resulting crystals are acicular with mean aspect ratios above 4:1; such needle-like crystals retain mother liquor by capillary action and lower the product melting point by 0.3 °C to 0.8 °C after filtration. Industrial experience on a 1.2 m² wiped-film exchanger with 304L stainless steel heat transfer surface and a glycol-water coolant at −10 °C showed that the overall heat transfer coefficient declined from 220 W/m²·K to 120 W/m²·K after 48 h when the wall temperature was 15 K below the bulk slurry setpoint. Reducing the coolant differential to 6 K and increasing the scraper speed from 60 rpm to 90 rpm extended the operating interval between warm-up cycles from 48 h to 120 h. The observed fouling layer consisted primarily of naphthalene with 6–9 wt% occluded methylnaphthalenes and 1–2 wt% biphenyl, as determined by dichloromethane extraction and gas chromatography. These impurities are not removed by ordinary filtration; they require either sweating at 70–75 °C or reslurrying in warm toluene.

Product purity after crystallisation and filtration is improved by sweating under a controlled reheating ramp that selectively melts impure crystal surfaces. In a sweating step, the filtered naphthalene cake is heated from 40 °C to 75 °C at 0.5 K/h under a nitrogen atmosphere in a closed vessel. The melted fraction from 40 °C to 68 °C typically contains 85–92 wt% naphthalene and is recycled to the crystalliser feed because it is rich in methylnaphthalenes. The remaining cake after sweat fraction removal can reach 99.0–99.5 wt% naphthalene if the crystallisation setpoint was kept above the secondary exotherm and the cooling ramp did not exceed 4 K/h. A vacuum belt filter with countercurrent wash of warm 60 °C naphthalene-saturated toluene at a wash ratio of 0.3–0.5 kg/kg cake reduces surface-adhered mother liquor from 4–6 wt% to 0.5–1.0 wt%. The wash solvent is recovered in a falling-film evaporator operated at 80 °C and 0.15 bar absolute, and the recovered solvent is returned to the wash circuit without exceeding 50 ppm water to avoid phase separation and ice-like hydrate fouling in the vacuum pump. Naphthalene purity is determined by gas chromatography with a flame ionisation detector using a 30 m nonpolar capillary column with 0.25 mm internal diameter and 0.25 µm film thickness, calibrated against certified reference materials under ISO 17025:2017; the reported repeatability for naphthalene content at 99 wt% is 0.1 absolute area percent. Melting point is measured by capillary melting point apparatus according to ASTM E324-23.

When Crystallisation Feedstock Contains More Than 15 wt% Methylnaphthalenes

When crystallisation feedstock contains more than 15 wt% methylnaphthalenes, the solid-liquid phase envelope narrows and the crystallisation setpoint must be lowered in a stepwise manner that accounts for increased impurity solubilisation. High methylnaphthalene concentrations reduce the activity coefficient of naphthalene in the liquid phase and therefore depress the primary crystallisation onset by 4 K to 7 K relative to a feed with 8 wt% methylnaphthalenes. The resulting crystals contain a higher fraction of substitutional inclusions, and the one-stage product purity from suspension crystallisation may fall from 97 wt% to 92 wt% if the reduction in setpoint is not accompanied by longer sweating. A suitable control response is to lower the first-stage crystallisation setpoint from 60 °C to 55 °C, the second-stage setpoint from 53 °C to 48 °C, and the third-stage setpoint from 46 °C to 42 °C, while increasing the sweating time at 70 °C by 30 min for each percentage point rise in alpha-methylnaphthalene above 12 wt%. The mother liquor viscosity at 40 °C increases from 2.5 mm²/s to 4.5 mm²/s, which reduces the filtration rate by 25–35% and requires a larger vacuum pump to maintain 0.4 bar differential. When beta-methylnaphthalene exceeds 8 wt%, the filter cake becomes compressible; at a normal stress of 0.6 bar the cake porosity falls below 0.35, blinding the cloth and requiring an additional reslurry washing step. These feed-specific effects are monitored by measuring feed composition by gas chromatography and by determining the crystallisation onset via ASTM E794-06(2018), and the setpoints are adjusted only after the batch-to-batch variation exceeds ±0.5 wt% naphthalene or ±0.3 wt% alpha-methylnaphthalene.

Cold spots in the crystalliser are the principal source of off-specification product when naphthalene recovery is driven by crystallisation temperature. A thermal imaging survey of an operating 10 m² scraped surface crystalliser showed temperature differences of 4 °C to 6 °C across the tube sheet due to uneven coolant distribution, and the colder tubes produced a scale layer that reduced tube-side flow velocity from 1.8 m/s to 1.2 m/s. The resulting hydraulic imbalance increased the coefficient of variation of outlet slurry temperature from 0.3 °C to 1.1 °C, and the naphthalene product melting point shifted from 79.8 °C to 79.2 °C. Corrective measures include calibrating the coolant distributor to limit tube-to-tube flow variation to ±5%, installing a static mixer downstream of the crystalliser, and avoiding feed moisture above 100 ppm. Oxygen ingress above 120 °C must be avoided because heavy C10+ aromatics form colour-forming gums that lower salable product quality. The crystallisation temperature control procedures described above are limited to feedstocks with naphthalene content between 15 wt% and 30 wt% and to bulk slurry temperatures below 75 °C; published data for feeds outside this range and for configurations using direct contact cooling are limited.