Methanol Column Pushed Past Its Hydraulic Limit After a Steam Curtailment
After a site-wide steam curtailment, the board of a methanol–water column was pushed: reflux was cut from 1.4 to 1.1 (R = L/D) and feed was raised from 12.0 to 13.5 t/h at 40 wt% methanol. Within 6 h the top product fell from 99.6 to 96.8 wt% and the bottoms rose from 0.08 to 0.90 wt%. Column ΔP rose from 0.32 to 0.58 bar, reboiler steam is 100% open, and the tray pressure survey shows the largest ΔP between trays 22 and 28 (feed is at tray 18). Condenser outlet temperature rose 8 °C, reflux drum level is steady, overhead pressure control is steady at 1.0 barg. Eight months ago an internal inspection found no fouling, and the column met specification at 12 t/h and R = 1.4 with 0.32 bar ΔP. Bottoms goes to a biological effluent plant with a 300 kg/day methanol load limit; the top product feeds a reactor that tolerates no more than 0.5 wt% water.
1. Diagnose the problem. Distinguish flooding, fouling and foaming, and explain what the pressure survey localises.
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Localisation. ΔP more than doubled (0.32 → 0.58 bar) and the survey puts the maximum gradient between trays 22 and 28, below the feed tray (18). That is the stripping section, where the vapour load is highest because the reboiler is flat out. Fouling would show a gradient distributed across the whole column and would not appear overnight; foaming shows erratic, oscillating ΔP with liquid carryover and unstable drum level. Here ΔP is high but steady, the drum level is steady (no gross entrainment), and the change is step-like and coincident with the throughput increase.
Most probable cause: flooding in the stripping section. The vapour rate rose 12.5% (more feed plus the reboiler wide open), and the downcomer load rose with it. Above the jet-flood point, liquid backs up in the downcomers, ΔP climbs steeply, and separation collapses - which is exactly the observed fall in top purity to 96.8% and rise in bottoms methanol.
The second, independent cause: the reflux cut. Dropping R from 1.4 to 1.1 reduces L/V in the rectifying section, so the top loses stages of separation at the same time. A rough Fenske check (xD 0.995, xB 0.001, α_avg ≈ 2.0, ~20 theoretical stages from 30 trays at ~70% efficiency) gives N_min ≈ 17.6 and N/N_min ≈ 1.14. Gilliland then requires R only ~10% above R_min, so R_min ≈ 1.25–1.30 - meaning R = 1.1 is at or below the minimum reflux for the specified separation. With α_avg uncertain by ±20% (methanol–water α falls from ~2.5 dilute to ~1.4 at high methanol), treat this as indicative: the direction is unambiguous, the margin is not.
Discriminator. Hold throughput and reflux at the old values and log ΔP: if it returns to 0.32 bar, the trays are clean and the problem is hydraulic, not fouling.
2. Close the material balance on the off-spec operation and quantify the environmental and quality consequences. How long do they have?
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Methanol in. 13.5 t/h × 0.40 = 5.40 t/h.
Split. With xD = 0.968 and xB = 0.009: D × 0.968 + (13.5 − D) × 0.009 = 5.40 0.968D + 0.1215 − 0.009D = 5.40 → 0.959D = 5.2785 → D = 5.50 t/h, B = 8.00 t/h.
Check. D + B = 13.50 t/h ✓; methanol out = 5.33 + 0.07 = 5.40 ✓. The analysers are consistent - this is a real loss, not an instrument artefact.
Environmental. Methanol to effluent = 8.00 × 0.009 = 72 kg/h = 1.73 t/day, against a 300 kg/day limit. Even if the biological plant sees this stream alone, they have under 4 hours of headroom before the limit is breached - and it is not alone. The excess over the previous operation (8.00 × 0.0008 = 6.4 kg/h) is about 66 kg/h.
Product quality. At 96.8 wt% the top product carries 3.2% water = 176 kg/h; at the 99.6% design it carried 22 kg/h. The reactor tolerance of 0.5 wt% allows only 27.5 kg/h. The column is feeding the reactor roughly six times its water tolerance, so off-spec material is either being diverted (and the yield loss is elsewhere) or the reactor is being damaged.
Time available. The environmental headroom is hours, not days, and the reactor feed quality is already violated. The decision window is the current shift.
3. Give the immediate corrective sequence, and then the operating envelope and alarms you would set to prevent recurrence.
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Immediate sequence (safety first, then separation).
- Divert the top product to the slops or recycle tank - stop feeding the reactor. Route the bottoms to hold or reduce the feed rate so the effluent load falls; notify the biological plant now.
- Cut the feed back toward 11–12 t/h while the reboiler is at 100% steam. Do not simply open the reflux: with the column already flooded, added reflux makes it worse. Reduce boil-up first, then restore R.
- Once ΔP starts to fall, bring R back to 1.4 and re-establish the top temperature/bottom composition profile. Confirm ΔP returns to ~0.32 bar at 12 t/h; if it stays high at the same throughput, stop and inspect - that would indicate fouling or damaged trays.
- Only then step the feed back up in 0.5 t/h increments, watching ΔP and the tray survey.
Operating envelope.
- Maximum feed 12.5 t/h at R ≥ 1.35; maximum vapour load expressed as a ΔP ceiling of 0.40 bar, with a high alarm at 0.38 and a trip/interlock on the reboiler at 0.45 bar.
- Minimum reflux interlock at 1.30 (near R_min, computed above) - the DCS should not accept a lower set point without a management-of-change approval.
- Bottom composition on-line analyser with a 0.3 wt% alarm; top water analyser with a 0.4 wt% alarm ahead of the reactor.
- Tray pressure survey trended, not just total ΔP, so the flooding zone is visible before the top product moves.
- Feed quality and preheat checked: a colder feed shifts the operating line and reduces the flooding margin.
Root cause for the MoC. The change was made as an operating decision under a steam constraint, without a hydraulic review. The column has no relationship between throughput and minimum reflux documented anywhere the board operator can see - that, not the steam, is the real failure.