20 % Debottleneck of a Fixed-Bed Hydrogenation Reactor With a Rising Pressure Drop
A fixed-bed reactor hydrogenates a diene impurity in a C4 raffinate over 3 mm trilobe nickel catalyst. The 2.4 m ID reactor holds 6 m of catalyst and runs at 25 bar with a 2:1 recycle. Design is 12 t/h fresh feed at 180 °C inlet with a 258 °C hot spot; the catalyst limit is 300 °C and the high-temperature trip is set at 290 °C. Current bed pressure drop is 1.6 bar against a 2.0 bar alarm, up from 0.8 bar at start-of-run. Commercial wants 14.4 t/h. The options on the table are: (a) raise the inlet temperature by 10 °C; (b) increase recycle; (c) use a larger catalyst particle in the top third of the bed; (d) build a second parallel reactor. Selectivity to the saturated by-product is 98.5 % today and falls about 0.6 percentage points per 10 °C above a 250 °C hot spot, and the by-product is a low-value stream. The recycle compressor has about 3 bar of headroom.
1. Evaluate option (a), a 10 °C inlet temperature increase, using the reaction kinetics and the heat balance of the bed.
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Kinetics: for an activation energy near 80 kJ/mol, a 10 °C rise at 180 °C multiplies the rate constant by about exp[9625 × (1/453 − 1/463)] ≈ 1.6. The reaction accelerates by roughly 60 % at the inlet, exactly where the reactant concentration is highest.
Heat balance: for an exothermic fixed bed with a near-adiabatic front section, the local heat release per unit length rises with that same factor. The adiabatic temperature rise of the stream is fixed by conversion, but the hot spot is a balance between heat generation and axial/radial removal. Experience with this geometry says a 10 °C inlet step moves the hot spot 15–20 K, i.e. from 258 °C to roughly 275–280 °C.
Consequences:
- The trip is at 290 °C and the catalyst limit is 300 °C. A 275–280 °C hot spot leaves 10–15 K, which is inside the combined uncertainty of thermocouple position, inlet temperature control and feed composition.
- Selectivity loses about 1.5 points (2.5 hot-spot decades above 250 °C × 0.6), which on 14.4 t/h is a meaningful low-value by-product stream that must be separated and disposed of.
- Thermal runaway margin falls sharply because the reaction rate is now exponentially sensitive at the point of highest concentration.
Verdict: (a) alone is not viable. The hot-spot margin and the selectivity penalty both breach normal operating limits, and it does nothing about the pressure drop, which alone blocks the throughput increase. A smaller step (5 °C) could be part of a package only after the pressure drop is fixed and the selectivity has been confirmed.
2. Assess options (b) and (c), and state the combination you would take forward with the evidence needed to justify it.
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Option (b), more recycle: recycle raises total flow through the bed. At 2:1 recycle, a 20 % increase in fresh feed raises the total flow about 20 %, and the Ergun equation gives roughly ΔP ∝ u for the viscous term and u² for the inertial term, i.e. a 1.2–1.44× multiplier. Current 1.6 bar becomes 1.9–2.3 bar, at or beyond the 2.0 bar alarm. The 3 bar of compressor headroom is consumed and the compressor duty rises with no selectivity benefit. It also lowers per-pass conversion, pushing more diene round the loop. Rejected as a fix, although a modest increase may be useful later to control the hot spot after the bed is modified.
Option (c), graded bed: the top third carries a disproportionate share of the pressure drop because it sees the highest mass velocity, the smallest effective voidage and the fouling front (fines, coke, crushed pellets). Replacing it with 6 mm trilobe in a graded sequence can plausibly cut total ΔP by 30–40 % (1.6 → 1.0–1.1 bar). At +20 % flow that is about 1.4–1.6 bar, inside the alarm.
The counter-risk is that larger pellets have less external area and lower heat-transfer area per unit volume, so a hot spot can form in the coarse zone. Mitigate by diluting the coarse layer with inert or by keeping the layer shallow.
Recommended package: graded bed + 5 °C inlet increase + confirmation of selectivity and hot spot by a pilot or by a validated reactor model, plus a thermocouple in the graded layer.
Evidence needed: the axial ΔP profile (top/middle/bottom taps) to confirm where the resistance sits, a sample of the top bed to distinguish fines migration from coking or crushing, and an Ergun-based prediction of the new bed at 14.4 t/h.
3. The trip at 290 °C is a safety instrumented function. What else must be verified before the operating envelope is changed?
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1. The trip is part of the safety case. The 290 °C trip protects against a hot spot that could exceed the catalyst limit and, ultimately, the mechanical design temperature and the relief case. Changing throughput, inlet temperature or bed configuration changes the rate at which the hot spot can rise and the time available for the trip to act. The trip's setpoint, sensor location, voting and proof-test interval must be revalidated against the new dynamics; a 10 K reduction in margin may make the existing proof-test interval indefensible.
2. Sensor coverage. A hot spot that moves upstream with a higher inlet temperature may sit outside the existing thermocouple positions. Verify placement before, not after, the change.
3. Relief and flare case. The reactor relief sizing assumes a particular heat-release scenario. If the new configuration can generate a higher peak heat release or a higher temperature at the relief point, the relief case must be re-run.
4. Downstream consequences. More by-product changes the recycle loop composition and the separation duty; the by-product may accumulate in the recycle and change the reactor feed.
5. Change control. The pressure-drop alarm and trip are operating limits in the permit and the operating envelope. Any movement of them is a management-of-change action requiring HAZOP review, updated operating procedures and operator training.
6. Mechanical integrity. A pressure-drop rise from 0.8 to 1.6 bar over the run is a symptom. Confirm whether the cause is fines migration, coke or crushing before adding flow, because the same mechanism may accelerate under the new conditions and lead to bed collapse and bypassing.