Solvent Swap to a Low-Flash-Point, Peroxide-Forming Solvent Under a "Minor" MOC
A specialty chemicals batch plant is switching a 12 m³ reactor from xylene (flash point 38 °C, boiling 138 °C) to THF (flash point −14 °C, boiling 66 °C) for a new product. The reactor sits in a closed building with a nitrogen blanket, a 316L jacket, a 90 kW agitator motor and a graphite rupture disc set at 4 bar g. Drums will be stored in the existing solvent store, and the reactor is charged by a portable drum pump through an open manway. The hazardous-area drawing shows Zone 2 around the reactor. The HAZOP was written for xylene, and the MOC has been raised as "minor - same volume, same equipment". The reactor high-temperature trip is set at 120 °C and the relief system was sized for xylene. The plant wants the first campaign in six weeks.
1. Assess whether this qualifies as a minor change and list the design-basis changes that must be re-evaluated.
Read the worked answer
It is not a minor change. The material hazard class changes fundamentally; the equipment is the same only in nameplate terms.
1. Area classification. A liquid with a flash point of −14 °C generates a flammable atmosphere at ambient temperature. The area around the reactor, the manway and the drum-pump connection moves from Zone 2 to Zone 1, and the inside of the vessel and the immediate vicinity of an open manway are effectively Zone 0. Existing motors, lighting, instrumentation and the portable pump must be suitable for the new zone. The 90 kW agitator motor is unlikely to be rated for Zone 1 as installed.
2. Charging method. Open-manway charging of a Category 1/2 flammable liquid is not acceptable. Closed charging, dry-break couplings, bonding and grounding, and control of static accumulation are required. THF is a poor conductor and a known static accumulator.
3. Pressure protection. THF's vapour pressure at the 120 °C trip is of the order of 3 bar g, only about 1 bar below the 4 bar g rupture disc. A trip failure or a slow excursion can burst the disc, which vents into a closed building. The trip setpoint, the disc rating and the relief scenario must all be re-derived; the fire case may no longer govern and a runaway reaction or vapour-pressure case may.
4. Chemistry and materials. THF forms peroxides on storage and attacks many elastomers. Gaskets, hoses, diaphragms and the drum-pump seals must be reviewed; inhibitor content and shelf life must be controlled; peroxide testing on receipt is mandatory.
5. Fire response. THF is water-miscible, so water spray spreads it and conventional foam may be ineffective. Alcohol-resistant foam and a revised emergency plan are required.
2. The site's LOPA spreadsheet shows the design meets the target by a factor of ten. Explain why this result may be misleading and what must be revalidated.
Read the worked answer
The arithmetic may be right while the inputs are invalid.
The typical credit structure is: initiating event (loss of cooling, 0.1/yr) × BPCS (0.1) × operator response to high temperature (0.1) × high-temperature SIF (0.01–0.03) × relief (0.01), giving 1e-7 to 3e-6 per year against a 1e-4 target. A factor of ten margin looks comfortable.
Where it breaks for THF:
- Operator credit. A 0.1 credit requires a response time well inside the process safety time. THF's low boiling point and the new reaction mean the time to maximum rate under adiabatic conditions may fall from hours to tens of minutes. If TMRad is shorter than the detection-plus-response time, the 0.1 must be removed. That alone moves the result by a factor of ten.
- SIF validity. The 120 °C trip is above the atmospheric boiling point of THF; the sensor range, the setpoint and the proof-test interval were chosen for xylene. If the trip cannot act before the relief device lifts, it is not an independent layer for this scenario.
- Relief credit. A relief device earns an IPL credit only if it is sized for the governing scenario and discharges to a safe location. Discharging into a closed building through a burst graphite disc is not a valid final element.
- Common cause. If the temperature sensor is shared with the BPCS, the BPCS and SIF credits are not independent.
Revalidation needed: a fresh HAZOP and LOPA for the THF basis, calorimetry (or literature data with justification) for the reaction and the solvent, a re-rated relief case, and a demonstrated operator response time. Until then the spreadsheet is a record of the xylene plant, not this one.
3. The business insists on running a first campaign in six weeks. State the conditions under which a limited campaign could proceed.
Read the worked answer
A limited campaign is defensible only as a formally approved temporary change with compensating measures, not as a shortcut. Conditions:
Technical
- Re-run the relief and vent-sizing for THF; verify the rupture disc rating and install a vent to a safe location or a scrubber.
- Lower the high-temperature trip to give a clear margin below the onset temperature derived from the calorimetry, and demonstrate the trip by test.
- Closed charging with dry-break coupling, bonding and grounding, and a conductive sample/charge line; no open manway while THF is present.
- Gas detection in the building interlocked to the agitator and to ventilation; verify the Zone 1 electrical suitability of the motor, lighting and portable pump, or remove them.
- Peroxide inhibitor verified on every drum, with receipt testing, date control and first-in-first-out; no drum older than the validated shelf life.
- Agitator interlock verified: loss of agitation is a credible runaway initiator.
Operational
- Reduce batch size to limit the inventory and the potential energy release (e.g. 50 % for the first campaign).
- Written emergency plan for an alcohol-resistant foam attack and for water-miscible spill containment; confirm the effluent route, since THF is water-miscible and volatile.
- Operator training specific to THF, with a documented competency check.
- Exclusion zone and permit-to-work control during charging.
Governance
- MOC reclassified from minor to major, with sign-off by the site process safety authority.
- Time-limited approval with defined success criteria and a mandatory review before any second campaign.