New Catalyst, Worse Conversion: A Fixed-Bed Hydrogenation Hot Spot
A single-train fixed-bed reactor hydrogenates nitrobenzene to aniline over a nickel-on-alumina catalyst. The reactor is 2.0 m inside diameter with 6.5 m of catalyst, cooled by boiling water on the shell side, and has no interbed quench. Feed is 9,500 kg/h nitrobenzene in recycle methanol with 1.2 times stoichiometric hydrogen; inlet is 90°C at 12 barg, and hydrogen is not limiting.
The original catalyst was 3.0 mm tablets, 12 wt% Ni, bulk density 850 kg/m³. It gave 0.8 bar bed pressure drop, a 135°C hot spot 2.0 m down the bed, 99.4% nitrobenzene conversion and 99.2% aniline selectivity. A second-source catalyst was loaded: 5.0 mm trilobes, 14.5 wt% Ni, higher BET area, bulk density 780 kg/m³. The vendor claims 25% higher intrinsic activity.
At the same feed rate and inlet temperature, the new load shows 0.45 bar pressure drop, a 158°C hot spot only 0.9 m down the bed, 97.0% conversion, 97.6% selectivity, and almost no temperature rise over the last 1.5 m. The aniline specification requires at least 99.0% conversion. Operations wants to raise inlet temperature to recover conversion.
1. The new catalyst is intrinsically more active, yet conversion and selectivity are worse. Give a physically consistent explanation for the pressure-drop, hot-spot and selectivity pattern, and rank the alternative causes you would still check.
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Start with the mass balance: if the bed were uniformly contacted, a catalyst with 25% higher intrinsic activity should increase conversion at constant feed, not reduce it by 2.4 points. The 44% lower pressure drop, the 158°C hot spot only 0.9 m into the bed, and the nearly isothermal last 1.5 m together suggest split flow: part of the gas passes through a high-activity path and overheats, while a significant fraction bypasses catalyst. Larger 5 mm trilobes can create wall voids and poor loading; the lower bulk density and shape also raise bed voidage. The hot path accelerates consecutive and coupling reactions that form azo/azoxy or amine byproducts, lowering selectivity, while the bypassed gas leaves unreacted and lowers overall conversion.
Intraparticle diffusion is real for 5 mm pellets, but by itself it would lower the effective rate and move the hot spot downstream, not upstream. It is a secondary contributor. Ranked causes: (1) maldistribution or channeling from loading, a damaged distributor, or wall bypass; (2) pore-diffusion limitation in the larger pellet; (3) feed poison or poor hydrogen distribution; (4) thermocouple placement error; (5) rapid catalyst deactivation. Do not raise inlet temperature; it will intensify the hot spot and make selectivity worse.
2. Design a diagnostic program to distinguish channeling from diffusion limitation and poisoning. Give the specific measurement and the result that would confirm each mechanism.
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(1) Radial temperature traverse at 0.5, 1.0, 2.0 and 4.0 m. A 10–20°C spread at the same elevation confirms channeling; a symmetric profile with a downstream hot spot points to kinetics or diffusion. (2) Gamma scan or neutron backscatter at the same elevations. A density anomaly or void confirms poor loading or a collapsed bed. (3) Sample whole pellets and crushed pellets from the hot-spot zone and the outlet, and test them in a lab microreactor at plant conditions. If the crushed sample is much more active than the whole pellet, diffusion limitation is significant; if both are dead, poisoning or sintering is indicated. (4) Tracer test or pressure drop versus flow: ΔP should rise with flow squared. A flat response suggests a fixed bypass. (5) Analyze feed for sulfur, CO, water and other poisons, and check hydrogen purity and distributor holes.
Channeling would be confirmed by a radial temperature spread, a gamma-scan void, and a pressure drop below the Ergun prediction. Diffusion limitation would be confirmed by a large crushed-versus-whole activity difference with a symmetric radial profile. Poisoning would show a dead catalyst at the inlet and a normal pressure drop.
3. What would you change for the next catalyst load, and what operating change would you make now?
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For the next load, return to a 3 mm pellet or use an eggshell catalyst with the active metal in a thin outer shell; this shortens the diffusion path and raises the effectiveness factor. If the more active 5 mm trilobe is kept, dilute the inlet 20–30% with inert, use a graded bed, improve loading with sock loading and a proper distributor, and add a radial thermocouple at 0.5 m. A graded bed with 3 mm in the top 2 m and 5 mm below balances activity and pressure drop.
Operating now: reduce inlet temperature to 85–88°C and cut feed to 7,500–8,000 kg/h until the hot spot is below 145°C; do not increase hydrogen. Check the outlet aniline specification; if off-spec, route to storage or recycle. Verify pressure drop versus flow to detect bypass. The tradeoff is that smaller particles raise ΔP; a 3 mm pellet may give 1.0–1.2 bar, so confirm the compressor or blower head before committing to the change.