1.2 Flammable Storage & Reactor Safety Flashcards
(13 cards)
Atmospheric storage
Cone roof tankage
Floating roof tankage
Cone roof tankage
Tank roof is fixed
There is always a vapour space above the liquid level - vapour space expands and contracts as liquid is pumped in
Used for storing materials at temp < flash point (approx 8 degrees away to avoid ignition)
Should have a frangible roof (weak shell to roof seam) so roof can come off but don’t want bottom to fail
Floating roof tankage
Tank roof floats on top of the liquid surface and rises/falls as the liquid level in the tank changes
There is no vapour space between the liquid and the roof (removes hazard)
Used for storing materials at temp > flash point
Not suitable for liquids with TVP (true vapour pressure) > 0.9 bara e.g butane as would start bubble (and cause roof collapse?)
Pressurised storage
Used to store materials that are vapour/gas at normal atmospheric conditions - too volatile to store in CR or FR tanks. (e.g butane NBP = -5 C, propane NBP = -45 C)
Storage container is designed as a pressure vessel:
- above ground sphere
- above ground drum (bullet)
- rounded drum (bullet)
Most VCE occur due to release of flashing liquids (flammable gas stored as liquid under pressure)
BLEVE
• In a fire scenario, pressurised storage has the potential to create a BLEVE (Boiling Liquid Expanding Vapour Explosion)
– Low overpressure (blast wave)
– Main hazard is due to radiant heat from fire ball (up to 500m)
• When vessel is exposed to fire the metal weakens. As liquid inside the vessel boils-off the vessel wall dry out and metal surface temperature increases
– Metal softens, yields and ruptures releasing expanding liquid vapour
• Key mitigation:
– Gas and fire detection
– Emergency Block Valves (EBV)
– Drencher system
– Containment area slopes away from sphere – Use of mounded drum (inherently safe)
LPG sphere BLEVE - Elf refinery El Feyzin (1966)
• Operator was draining water from the sphere to local sewer
• Valve partially blocked due to hydrate formation (sub-zero temperature due to auto refrigeration)
• Blockage suddenly cleared when valve opened fully
• Leak ignited 25 min later by car travelling on nearby road (150m)
• 90 min after fire started sphere BLEVE occurred
- 15 killed; 80 injured
Batch and semi-batch reactors
• Used extensively in speciality chemicals and pharmaceuticals industries
– Low volume
– Facilities sometimes used to produce different grades/products
Hazards of runaway exothermic reactions
– Potential for rapid thermal decomposition causing deflagration/detonation (e.g. ethylene oxide)
– High bulk temperature can cause material to boil/vaporise. Potential for contents to overpressure and erupt from vessel.
– Reaction generates high volumes of gas which overpressures the reactor
– Secondary fire/explosion due to loss of primary containment
Causes of runaway reactions
- Reactive chemistry and transition to other unwanted reactions not understood
- Reactants added in the wrong quantities or order
- Contaminants
- Temp control inadequate
- Poor mixing (mixer not started or stopped during the reaction)
- Inadequate emergency venting failures
- Failure to take emergency action in the event of high temp
Effect of scale-up on heat balance
Rate of heat production is proportional to volume
Natural cooling capacity proportional to surface area
Reactive chemistry
- Literature search to identify industry experience and lab data
- Conduct calorimetric tests using in-house research facilities or external lab specialists
- Oxygen balance can help identify whether CxHyOz compound could decompose violently
CXHYOZ +(2X+Y/2–Z)O->XCO2+Y/2H2O
Oxygen balance = -1600(2X + Y/2 – Z) ÷ MW
(High Risk if > -200) - From heat of reaction can estimate the max adiabatic temp rise (= heat of reaction/Cp)
- Check if max temp is below temp at which: other reactions start to take place (e.g decomposition), reactants boil, gas evolution occurs
Runaway exothermic control measures
- Emergency cooling facilities
- Chemical inhibitor injection to suppress reaction or poison catalyst
- Drown-out or quenching; use an inert medium to quench and dilute the reactants, may need to dump contents to a secondary vessel if insufficient space in reactor.
- Provide adequately sized emergency venting facilities (bursting disc). Consider hazards of venting reactor to atmosphere and need for scrubber tower/containment facility
- Protective instrument systems can be used to automate some or all of the above
- Consider an inherently safer design: use semi-batch operation and add reactants gradually, use CSTR, use smaller reactor volume (e.g loop type reactor), design reactor to withstand worst case temp/pressure conditions.
Reactors in refineries
Protective measures
• Although reactions are mostly exothermic (Hydrofining, Polymerisation, Alkylation) these are typically less severe
• Process is continuous typically using fixed bed heterogeneous catalytic reactors (e.g. Naphtha Reformer)
• Reaction is vapour or 2-phase flow and units are therefore operated at moderate to high pressure (20-150 barg) and high temperature (300-400 deg C)
• As temperature increases there is potential for basic reactions to transition to more exothermic reactions (e.g. hydrocracking)
• Protective controls include:
– Use of emergency inter bed cold quench – Emergency depressurisation
⇒ Reduces partial pressure of reactants
⇒ Reduces stresses on vessel to prevent rupture in the event of over-temperature
