Hickson Accident

1. Hickson Accident Ringaskiddy, Co. Cork 6th August 1993 An explosion occurred in a batch chemical reactor in a contract pharmaceutical plant. While damage to the plant and wider environment was extensive, there were no fatalities. . Thanks to Ann-Marie McSweeney, John Barrett & Jacinta Sheehan Ware Department of Process Engineering, UCC

2. Hickson Accident Incident Overview An explosion occurred in a batch chemical reactor that was designed to recover Isopropyl Alcohol from waste products from a chemical reaction. An exothermic decomposition of the mixture produced too much gas / vapour which the vessel bursting disc was unable to relieve. Thus although the vessel bursting disc opened, the pressure within the reactor vessel continued to rise. The membrane stress in the reactor wall exceeded the tensile strength of the wall and the reactor burst open. Fragments from the burst reactor punctured adjacent vessels and their flammable contents spilt out. A large fire then engulfed that section of the plant.

3. Hickson Accident A very good description of the incident is given in the Health & Safety Authority Report of Investigation The course notes for PE 2002 should also be consulted especially the material dealing with pressure vessels.

4. Hickson Accident AERIAL VIEW OF INCIDENT

5. Hickson Accident PROCESS BACKGROUND Hickson PharmaChem Ltd. owned the site and used it for the contract manufacture of pharmaceutical and chemical intermediate products. The factory was located at Loughbeg, Ringaskiddy. The explosion occurred in a batch chemical reactor in the Isopropyl Alcohol recovery section of the factory. The main process equipment in this section included: • Chemical Reactor RE 02-05 • Distillation Column • Condenser • Recovered IPA Tank

6. Hickson Accident BATCH REACTOR WITH DISTILLATION PLANT

7. Hickson Accident PROCESS DESCRIPTION IsoPropyl Alcohol (IPA) is a common solvent used in pharmaceutical processing to aid in the synthesis of products. After a batch reaction is completed the IPA solvent is contaminated with chemicals and must be purified for re-use or if not destroyed by incineration. So IPA recovery had a double benefit for the company by reducing both incineration charges and the cost of buying new solvent. Recovery was accomplished by charging a vessel (Reactor RE 02-05) with the contaminated liquor (whose primary component was IPA) and then separating the IPA from the liquid by distillation; i.e. ‘boiling off the IPA’. The residue of the liquor left in the reactor (consisting of solid and liquid waste) could then be sent for incineration.

8. Hickson Accident SCHEMATIC PLANT LAYOUT Condenser IPA Vapour IPA Liquid IPA Collection Tank Distillation Column Reactor

9. Hickson Accident PRODUCT DESCRIPTION Isopropyl Alcohol (IPA) C3H8O or CH3CHOHCH3 Molecular Weight M = 60 Gas Constant R = 138.6 J/kgK Boiling Point at Patm is 82 °C (i.e. it is liquid at room temperature).

10. Hickson Accident PRODUCT DESCRIPTION Compare properties of IPA (similar to Propanol) to Ethanol which is the most common member of the alcohol family. The above data is for the liquid at Patm and 20 °C

11. Hickson Accident VAPOUR PRESSURE CURVE FOR IPA T °C 0 20 40 60 80 100 110 P bar (abs.)0.01 0.04 0.15 0.4 0.94 2.0 2.8

12. Hickson Accident CONTAINMENT DESCRIPTION Batch chemical reactor given the identification RE 02-05 Reactor Description Vertical cylindrical vessel with dished heads Diameter D = 2.0 m Height H = 2.5 m Vessel Volume V = 7.85 m3 Wall thickness (of the cylindrical sides), t = 8 mm There was an agitator present in reactor. The reactor cylindrical shell had a coil to heat or cool the reactor contents; the heating fluid was hot water at 80 °C The cylindrical vessel sides and bottom were lagged with thermal insulation.

13. Hickson Accident REACTOR MECHANICAL DESIGN Designed to Pressure Code BS5500 Category 2 Design Pressure: Full vacuum to 1.5 bar Design Temperature: -15 °C to +130 °C Vessel was fitted with a 150 mm diameter bursting disc set to rupture at 1.5 bar gauge. The hoop stress in cylindrical walls of the reactor at maximum pressure Pmax is = 18.75 MN/m2

14. Hickson Accident REACTOR MECHANICAL DESIGN Material of construction is stainless steel, grade unknown. Assume an 18/8 stainless steel. The following are indicative of the mechanical strength properties. σTS= 540 MN/m2 σPROOF = 200 MN/m2 Estimating rupture pressure i.e. the vessel internal pressure that will produce fracture. = 43 bar based on σTS = 16 bar based on σPROOF

15. Hickson Accident NORMAL PROCESS OPERATION Aim was to recover IPA solvent by batch distillation of waste liquor from the plant. • Charge the vessel with 7 m3 of waste liquor (mostly IPA plus mixture of organic compounds) • Start agitator and drop pressure to 0.2 bar absolute. • Run hot water at 80 °C through the coil which heats mixture up to about 50 °C • The IPA vapour boils off and is sent into distillation column • After approximately 5.5 m3 of original charge is removed, stop distillation by turning off hot water and running coolant through coil • Bring pressure back up to Patm and remove residue of 1.5 m3 of liquid (this is subsequently sent to an incinerator)

16. Hickson Accident INCIDENT DESCRIPTION The known or inferred facts are: At the end of a distillation sequence, residue was left in the reactor comprising of; • 1.5 m3 of IPA • 500 kg of various organic intermediate liquid products. This would imply that the reactor was just 25 % full. The residue was left in the reactor for 6 days during which the coolant and agitator should have been left on, but both were interrupted.  An exothermic chemical reaction began in the residue generating gases and IPA vapour. At 7.00 a.m. 06/08/1993 P = 1 bar abs. (i.e. Patm) and T = 42 °C in RE 02-05 At 7.13 a.m. 06/08/1993 RE 02-05 exploded

17. Hickson Accident SCHEMATIC VIEW OF BATCH REACTOR

18. Hickson Accident INCIDENT DESCRIPTION • The initial explosion occurred in the chemical reactor causing metal fragments from this vessel to puncture other vessels that contained flammable liquids. • These liquids spilled onto the ground, ignited and fires began. • The radiant heat from these fires caused the liquids held in adjacent storage tanks to boil and pressurize these tanks. • These tanks were no able to vent their vapours in a safe manner in this situation of fire engulfment. • They in turn ruptured, spilt their flammable liquids and added to the fire. The immediate area was soon engulfed in a large fire.

19. Hickson Accident AREA AFTER THE EXPLOSION

20. Hickson Accident INCIDENT DESCRIPTION The loss of Containment (vessel rupture) did not directly produce major environmental damage. • To contain and then quench the fire, large amounts of fire fighting water were used. • The spent fire water was diverted to a special retention pond as it was contaminated with various chemicals. • Subsequently some of this contaminated water overflowed into Cork Harbour through an overflow pipe. • To limit this spillage as much of the fire water as possible was redirected to any available tanks; nonetheless environmental damage did occur.

21. Hickson Accident ACCIDENT ANALYSIS Subsequent metallurgical inspection of RE 02-05 showed failure was not due to poor design or construction, but rather due to internal over-pressurisation. Source of Pressurisation • Not hydraulic rupture due to over-filling because rupture of the bursting disc would have permitted liquid expansion. • Not due to some high pressure external gas inadvertently entering vessel. Answer Gas/vapour must have been generated at a rate greater than the bursting disc could relieve: thus pressure rose to Prupture.

22. Hickson Accident ACCIDENT ANALYSIS The pressure build-up in the vessel that caused the vessel to rupture had to occur at a faster rate than the pressure relieving system of the reactor could adequately vent. Two questions follow: • What caused the build-up in pressure to occur? • Why wasn’t the pressure relieving capacity sufficient? No definitive answer to these two questions.

23. Hickson Accident REACTOR PRESSURE BUILD-UP The solid residue was chemically unstable and could exothermically decompose. Consequently heat would be liberated at an extremely fast rate. This heat would then vapourize the liquid in the vessel and the rate of vapourization exceeded the pressure relief capacity of the venting system. Two design aspects of the reactor helped to allow this highly unstable decomposition: • The cooling coil on the external reactor wall did not extend fully to the base of the vessel so heat transfer (i.e. cooling) would have been poor in this region. • The level of the residue solution in the reactor was below the bottom of the agitator so that mixing, which aids heat transfer, would have been ineffective.

24. Hickson Accident REACTORVESSEL RELIEF RATE The mass of IPA as a function of time after the bursting disc opened is given as m(t) Mass of product in reactor kg mi Initial mass in reactor (at the point the disc blows) kg JREL Rate of vapour outflow through disc kg/s Initial mass of IPA is approximately: mi = volume x density = 1.5 m3 x 790 kg/m3 = 1185 kg

25. Hickson Accident REACTORVESSEL RELIEF RATE The mass flowrate through the disc can be found by treating the flow as single phase, ideal gas, compressible, sonic flow P Reactor Pressure Pa T Reactor Temperature K A Disc flow area m2 R IPA Gas Constant J/kgK γ IPA Ratio of Specific Heats -

26. Hickson Accident REACTORVESSEL RELIEF RATE P Reactor Pressure 2.5 bar abs. = 250000 Pa T Reactor Temperature 60 C = 333 K A Disc flow area (π/4) x 0.152 = 0.018 m2 R IPA Gas Constant 138.6 J/kgK γ IPA Ratio of Specific Heats 1.2 (assumed value)

27. Hickson Accident POSSIBLE ACCIDENT PREVENTION STRATEGIES The incident could have been prevented or its magnitude minimized if: • An automatic cooling system had been installed on the reactor. • Provision had been made for dilution or quenching of the reactor contents with an suitable fluid. • Reactor RE 02-05 had been designed to withstand the pressures that could be generated during thermal runaway reactions. • A transfer system had been configured to move rapidly the contents of the reactor to a larger vessel • Adequate pressure relieving capacity to vent the generated gases had been selected. • Storage tanks containing toxic or flammable materials had not been located so closely together.