Propionic Acid Plant Audit: 8 On-Site Checks That Replace Factory Photos begins with the recognition that exterior photography cannot verify feedstock quality, instrument calibration, pressure-drop trends, or alloy composition. Propionic acid, CAS 79-09-4, boiling point 141.2 °C at 101.3 kPa, density 0.993 g/cm³ at 20 °C, pKa 4.87, and closed-cup flash point 54 °C, is processed through oxidation, distillation, and corrosion-sensitive storage equipment where process state must be read directly.

The matrix below identifies the checks, the direct measurement, the primary reference, and the external photo limitation.

At the propionaldehyde day tank discharge line, the first check replaces a site photograph of the tank with a grab sample taken after circulation through the transfer loop. The sample is analyzed for water by ASTM E203-16, for active oxygen species by ASTM E298-17a, and for total acidity by ASTM D1613-17. Aged propionaldehyde can accumulate peroxides without visible colour change, and the measured values are compared against the supplier specification and the plant quality plan rather than a universal acceptance threshold. Published process design packages for propionaldehyde-to-propionic acid oxidation typically identify the feed quality limits that protect oxidation selectivity and downstream equipment; where site data are missing, no photograph can supply the absent analytical result.

Is Reactor Vent Oxygen Analysis Reading the Same Value at the Fixed Transmitter and the Sample Point?

The reactor vent sample is drawn through a portable paramagnetic oxygen analyzer with an upstream condensate knock-out pot and a filtered sample line. The reading is compared with the fixed analyzer, and the instrument is challenged with certified calibration gas procured under ISO 6141:2015. Published safety data sheets commonly list the lower explosive limit of propionic acid near 2.9 % by volume; the oxygen measurement is therefore evaluated against the site’s process safety basis rather than a single generic oxygen limit. The audit verifies that the fixed oxygen analyzer is proof-tested within the interval defined in the site functional safety management system consistent with IEC 61511-1:2016. Exterior reactor photographs record insulation staining and transmitter housing condition, but they cannot detect calibration drift, sample-line condensation, or a rising headspace oxygen trend.

Because oxidation of propionaldehyde to propionic acid is exothermic, the coolant-loop check records the cooling water circuit instead of relying on reactor skin thermography. DCS trends are retrieved for cooling water inlet and outlet temperatures, coolant flow, and coil differential pressure. Temperature elements are checked against a reference platinum resistance thermometer conforming to IEC 60751. The overall heat transfer coefficient is calculated and compared with the clean-service baseline at the same production rate; a declining coefficient at constant throughput indicates coil fouling or partial blockage. The check also records agitator seal flush pressure and mechanical seal leak-off where mixing is provided by a top-entering agitator. A thermal image of the reactor shell cannot separate process exotherm from insulation defects or local steam tracing interference.

Distillation Column Pressure Drop and Reboiler Fouling Evidence

The pressure-drop check begins at the distillation column differential-pressure transmitter and ends with a product fraction sample. The measured pressure drop across each packed-bed section is compared with the vendor curve for the installed packing at the current boil-up rate and reflux ratio. A sustained rise above the clean-bed baseline indicates liquid maldistribution, polymerized heavy ends, or fouled distributor orifices. The reboiler steam pressure and condensate pH are logged because acidic condensate can indicate a tube leak or carryover. Product samples are analyzed for distillation range by ASTM D1078-16, density by ASTM D4052-18a, and water by ASTM E203-16. The boiling point of propionic acid is 141.2 °C at 101.3 kPa; a broad distillation range above this value indicates heavy-end carryover or polymerized material. External column photographs do not show pressure-drop drift or reboiler scaling.

Catalyst Fines Filtration Checked Directly at the Guard Bed

In oxidation processes using soluble catalyst salts or entrained metal species, post-reactor guard bed filters remove fine solids before distillation. The check measures differential pressure across the filter housing with a calibrated differential-pressure transmitter and records backwash interval, backwash solids loading, and element condition during a scheduled removal. Filter element retention rating is compared with the process design package; published data for this specific configuration is limited, so no universal numerical threshold is applied. The audit also checks that the backwash receiver is drained to the correct hazardous waste or process recovery destination. A photograph of a clean filter housing exterior does not document the pressure-drop trajectory or the remaining solids-holding capacity of the elements.

Exterior photographs do not distinguish 304 stainless steel from 316L stainless steel at the storage area. The on-site check uses handheld X-ray fluorescence to confirm alloy identity on tank shells, pump casings, and piping, and the material test reports are reviewed against ASTM A240/A240M-20a. Carbon steel storage is not acceptable for long-term propionic acid service at ambient temperature because of acid corrosion; 316L (UNS S31603) is commonly specified. At elevated temperatures or with chloride contamination, Alloy 20 (UNS N08020) may be required. Weld heat tint on stainless steel is also inspected because heat tint can increase local corrosion susceptibility. Immersion corrosion data generated to ASTM G31-72(2017) can support material changes where process conditions exceed the original design basis.

When Magnetic Drive Pumps Obscure Seal Leakage in Propionic Acid Transfer

Sealless magnetic drive pumps do not expose the process fluid through a visible mechanical seal leak, so an exterior photo of a clean pump skirt cannot validate hydraulic condition. The check records bearing wear indicator readings, containment shell temperature, motor power draw, flow, suction pressure, and discharge pressure, then compares the operating point with the pump curve and API 685 baseline. The fluid temperature is logged because the viscosity of propionic acid at 25 °C is approximately 1.1 mPa·s; lower temperatures increase viscosity and can overload the magnetic coupling if the minimum flow bypass line does not open at its setpoint. The audit also verifies that the pump is not operating against a closed discharge valve, which can cause internal heating and rapid bearing wear.

At the conservation vent and scrubber recirculation loop, the final check removes the flame arrestor element and verifies nitrogen blanket integrity. Nitrogen supply pressure and conservation vent setpoints are checked against API 2000. Flame arrestor pressure drop is measured with a manometer at the current vent flow, and the element is inspected for crystalline deposits or polymer film. The scrubber recirculation pump flow and discharge pressure are logged, and the scrubber solution pH is measured by ASTM D1293-18. Because the closed-cup flash point of propionic acid is 54 °C, a failed nitrogen blanket or blocked vent can create a flammable headspace condition that is not visible from outside the tank.