Solvent Recovery Column with Rising Pressure Drop and Falling Purity

Credible causes.

1. Tray fouling or scaling. Gradual rise in differential pressure over five months at constant throughput, with efficiency falling, is the classic fouling signature. Corrosion products, polymer or carried-over salts deposit on valves and downcomers.
2. Flooding. Rising boilup with a fixed top pressure produces downcomer backup, high differential pressure and entrainment that carries water overhead and drops purity. Consistent with the reboiler being nearly wide open.
3. Non-condensable accumulation. The vent is nearly shut. Inerts build up in the accumulator, blanket the condenser tubes and reduce the effective area. The visible fingerprint is exactly what is recorded: condenser outlet subcooling rising from 0 to 8 K, column pressure creeping up against a nearly closed vent, and reboiler duty climbing to hold the boilup.
4. Condenser cooling-water side fouling. Cooling water at 120 m3/h and 8 K rise removes about 1,115 kW (120 x 1000/3600 x 4.18 x 8). Against a design duty of 1,900 kW, that is a 41% shortfall, so the condenser, not the trays, is limiting the boilup.

Discriminating measurements.

• Gamma scan or a pressure-profile survey: differential pressure concentrated in the top section points to fouling or a flooded condenser; distributed across all trays points to flooding or general fouling.
• A boilup perturbation test: reduce reboiler duty by 10% and watch differential pressure. In flooding, differential pressure falls sharply; in fouling, it barely moves.
• Overhead vapour temperature against accumulator liquid temperature gives the subcooling directly.
• Sample the accumulator vent for methanol and inerts; check the condenser cooling-water outlet temperature trend at constant flow.
• Check reboiler steam flow and condensate rate against the duty needed for the distillate and reflux.

Condenser duty check. Q = 120 m3/h x 1000 kg/m3 / 3600 s/h x 4.18 kJ/kg.K x 8 K = 1,115 kW. Design duty is 1,900 kW, so the condenser is delivering about 59% of design.

Is that enough for the separation? The overhead vapour is roughly the distillate plus the reflux. At reflux ratio 1.4, with a distillate around 2,700 kg/h, the vapour to be condensed is about 6,500 kg/h. Methanol latent heat is about 1,100 kJ/kg, so the required duty is roughly 6,500 x 1,100 / 3,600 = 1,990 kW. The condenser is therefore short by several hundred kW regardless of how the trays behave.

What that does to the column. A condenser that cannot condense the vapour forces the column pressure up or the boilup down. Pressure control is trying to hold 1.05 bara by closing the vent, so inerts stay in the shell and the effective condensing area shrinks further. As pressure rises, relative volatility of methanol and water falls, purity drops, and the operator raises reflux to compensate, which adds more vapour to a condenser that is already limiting. The whole loop is self-reinforcing.

Conclusion. The condenser is the primary bottleneck; inerts and cooling-water side fouling are the likely causes. Tray fouling contributes to the differential pressure rise but cannot explain a 41% duty shortfall with 8 K subcooling.

Do first. Open the vent and purge the inerts to a safe location; check cooling-water flow, strainer and outlet temperature; clean the condenser (mechanical or chemical) and re-test duty. Only then review the trays.

Do not. Do not raise reflux further to chase purity, and do not raise steam. Both push more vapour into a condenser that cannot handle it and take the column closer to flooding and overpressure.

Corrective actions, in sequence.

1. Vent the inerts from the accumulator through the existing vent line to a safe location, with the receiver gas monitored for methanol and oxygen. Restore automatic periodic venting rather than relying on the operator, and check the vent line for blockage.
2. Restore condenser duty. Verify cooling-water flow (120 m3/h is roughly 0.55 m/s in a 250 mm line, so check for a partly closed valve or a blocked strainer). Clean the tubes and re-check the duty by the cooling-water heat balance, not by the outlet temperature alone.
3. Only then assess the trays. If differential pressure does not fall after the condenser is restored, plan a shutdown for inspection with a gamma scan beforehand to decide which sections to open.

Safety controls that must be in place throughout.

• The column relief valve and rupture disc must be sized for the case 'condenser fails at full reboiler duty'. With the condenser currently at 59% of design, the margin is already thin; verify the PSV data sheet before raising throughput.
• Methanol is flammable and toxic. Check the low-level gas detection around the accumulator, drum and pump seals before opening the vent, and confirm the vent discharges to a safe location.
• Confirm the high-pressure trip and the reboiler steam isolation valve are tested, and add a high-differential-pressure alarm so the next event is caught at 0.40 bar, not 0.58 bar.

Prevention.

• Trend the condenser heat balance continuously, not just the outlet temperature; a duty trend catches fouling weeks earlier.
• Write a venting procedure into the operating manual, and include inerts in the shift checklist.
• Check the cooling-water treatment programme; the subcooling rise suggests both inerts and biological or scale fouling on the water side.
• Raise a management-of-change record for the revised vending and cleaning interval.